clk: imx: Fix division by zero warning on pfdv2

Fix below division by zero warning:

[    3.176443] Division by zero in kernel.
[    3.181809] CPU: 0 PID: 88 Comm: kworker/0:2 Not tainted 5.3.0-rc2-next-20190730-63758-ge08da51-dirty #124
[    3.191817] Hardware name: Freescale i.MX7ULP (Device Tree)
[    3.197821] Workqueue: events dbs_work_handler
[    3.202849] [<c01127d8>] (unwind_backtrace) from [<c010cd80>] (show_stack+0x10/0x14)
[    3.211058] [<c010cd80>] (show_stack) from [<c0c77e68>] (dump_stack+0xd8/0x110)
[    3.218820] [<c0c77e68>] (dump_stack) from [<c0c753c0>] (Ldiv0_64+0x8/0x18)
[    3.226263] [<c0c753c0>] (Ldiv0_64) from [<c05984b4>] (clk_pfdv2_set_rate+0x54/0xac)
[    3.234487] [<c05984b4>] (clk_pfdv2_set_rate) from [<c059192c>] (clk_change_rate+0x1a4/0x698)
[    3.243468] [<c059192c>] (clk_change_rate) from [<c0591a08>] (clk_change_rate+0x280/0x698)
[    3.252180] [<c0591a08>] (clk_change_rate) from [<c0591fc0>] (clk_core_set_rate_nolock+0x1a0/0x278)
[    3.261679] [<c0591fc0>] (clk_core_set_rate_nolock) from [<c05920c8>] (clk_set_rate+0x30/0x64)
[    3.270743] [<c05920c8>] (clk_set_rate) from [<c089cb88>] (imx7ulp_set_target+0x184/0x2a4)
[    3.279501] [<c089cb88>] (imx7ulp_set_target) from [<c0896358>] (__cpufreq_driver_target+0x188/0x514)
[    3.289196] [<c0896358>] (__cpufreq_driver_target) from [<c0899b0c>] (od_dbs_update+0x130/0x15c)
[    3.298438] [<c0899b0c>] (od_dbs_update) from [<c089a5d0>] (dbs_work_handler+0x2c/0x5c)
[    3.306914] [<c089a5d0>] (dbs_work_handler) from [<c0156858>] (process_one_work+0x2ac/0x704)
[    3.315826] [<c0156858>] (process_one_work) from [<c0156cdc>] (worker_thread+0x2c/0x574)
[    3.324404] [<c0156cdc>] (worker_thread) from [<c015cfe8>] (kthread+0x134/0x148)
[    3.332278] [<c015cfe8>] (kthread) from [<c01010b4>] (ret_from_fork+0x14/0x20)
[    3.339858] Exception stack(0xe82d5fb0 to 0xe82d5ff8)
[    3.345314] 5fa0:                                     00000000 00000000 00000000 00000000
[    3.353926] 5fc0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
[    3.362519] 5fe0: 00000000 00000000 00000000 00000000 00000013 00000000
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

drivers/clk/imx/clk-pfdv2.c

@@ -139,6 +139,12 @@ static int clk_pfdv2_set_rate(struct clk_hw *hw, unsigned long rate,
 	u32 val;
 	u8 frac;
 
+	if (!rate)
+		return -EINVAL;
+
+	/* PFD can NOT change rate without gating */
+	WARN_ON(clk_pfdv2_is_enabled(hw));
+
 	tmp = tmp * 18 + rate / 2;
 	do_div(tmp, rate);
 	frac = tmp;