watchdog: omap_wdt: fix null pointer dereference

Fix issue from two patches overlapping causing a kernel oops

[ 3569.297449] Unable to handle kernel NULL pointer dereference at virtual address 00000088
[ 3569.306272] pgd = dc894000
[ 3569.309287] [00000088] *pgd=00000000
[ 3569.313104] Internal error: Oops: 5 [#1] SMP ARM
[ 3569.317986] Modules linked in: ip6t_rpfilter ip6t_REJECT nf_reject_ipv6 xt_conntrack ebtable_filter ebtable_nat ebtable_broute bridge stp llc ebtables ip6table_security ip6table_raw ip6table_nat nf_conntrack_ipv6 nf_defrag_ipv6 nf_nat_ipv6 ip6table_mangle ip6table_filter ip6_tables iptable_security iptable_raw iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 nf_nat nf_conntrack iptable_mangle musb_dsps cppi41 musb_hdrc phy_am335x udc_core phy_generic phy_am335x_control omap_sham omap_aes omap_rng omap_hwspinlock omap_mailbox hwspinlock_core musb_am335x omap_wdt at24 8250_omap leds_gpio cpufreq_dt smsc davinci_mdio mmc_block ti_cpsw cpsw_common ptp pps_core cpsw_ale davinci_cpdma omap_hsmmc omap_dma mmc_core i2c_dev
[ 3569.386293] CPU: 0 PID: 1429 Comm: wdctl Not tainted 4.3.0-0.rc7.git0.1.fc24.armv7hl #1
[ 3569.394740] Hardware name: Generic AM33XX (Flattened Device Tree)
[ 3569.401179] task: dbd11a00 ti: dbaac000 task.ti: dbaac000
[ 3569.406917] PC is at omap_wdt_get_timeleft+0xc/0x20 [omap_wdt]
[ 3569.413106] LR is at watchdog_ioctl+0x3cc/0x42c
[ 3569.417902] pc : [<bf0ab138>]    lr : [<c0739c54>]    psr: 600f0013
[ 3569.417902] sp : dbaadf18  ip : 00000003  fp : 7f5d3bbe
[ 3569.430014] r10: 00000000  r9 : 00000003  r8 : bef21ab8
[ 3569.435535] r7 : dbbc0f7c  r6 : dbbc0f18  r5 : bef21ab8  r4 : 00000000
[ 3569.442427] r3 : 00000000  r2 : 00000000  r1 : 8004570a  r0 : dbbc0f18
[ 3569.449323] Flags: nZCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment none
[ 3569.456858] Control: 10c5387d  Table: 9c894019  DAC: 00000051
[ 3569.462927] Process wdctl (pid: 1429, stack limit = 0xdbaac220)
[ 3569.469179] Stack: (0xdbaadf18 to 0xdbaae000)
[ 3569.473790] df00:                                                       bef21ab8 dbf60e38
[ 3569.482441] df20: dc91b840 8004570a bef21ab8 c03988a4 dbaadf48 dc854000 00000000 dd313850
[ 3569.491092] df40: ddf033b8 0000570a dc91b80b dbaadf3c dbf60e38 00000020 c0df9250 c0df6c48
[ 3569.499741] df60: dc91b840 8004570a 00000000 dc91b840 dc91b840 8004570a bef21ab8 00000003
[ 3569.508389] df80: 00000000 c03989d4 bef21b74 7f5d3bad 00000003 00000036 c020fcc4 dbaac000
[ 3569.517037] dfa0: 00000000 c020fb00 bef21b74 7f5d3bad 00000003 8004570a bef21ab8 00000001
[ 3569.525685] dfc0: bef21b74 7f5d3bad 00000003 00000036 00000001 00000000 7f5e4eb0 7f5d3bbe
[ 3569.534334] dfe0: 7f5e4f10 bef21a3c 7f5d0a54 b6e97e0c a00f0010 00000003 00000000 00000000
[ 3569.543038] [<bf0ab138>] (omap_wdt_get_timeleft [omap_wdt]) from [<c0739c54>] (watchdog_ioctl+0x3cc/0x42c)
[ 3569.553266] [<c0739c54>] (watchdog_ioctl) from [<c03988a4>] (do_vfs_ioctl+0x5bc/0x698)
[ 3569.561648] [<c03988a4>] (do_vfs_ioctl) from [<c03989d4>] (SyS_ioctl+0x54/0x7c)
[ 3569.569400] [<c03989d4>] (SyS_ioctl) from [<c020fb00>] (ret_fast_syscall+0x0/0x3c)
[ 3569.577413] Code: e12fff1e e52de004 e8bd4000 e5903060 (e5933088)
[ 3569.584089] ---[ end trace cec3039bd3ae610a ]---

Cc: <stable@vger.kernel.org> # v4.2+
Signed-off-by: Peter Robinson <pbrobinson@gmail.com>
Acked-by: Lars Poeschel <poeschel@lemonage.de>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>

drivers/watchdog/omap_wdt.c

@@ -205,7 +205,7 @@ static int omap_wdt_set_timeout(struct watchdog_device *wdog,
 
 static unsigned int omap_wdt_get_timeleft(struct watchdog_device *wdog)
 {
-	struct omap_wdt_dev *wdev = watchdog_get_drvdata(wdog);
+	struct omap_wdt_dev *wdev = to_omap_wdt_dev(wdog);
 	void __iomem *base = wdev->base;
 	u32 value;