xhci: Fix Kernel oops in xhci dbgtty

tty_unregister_driver may be called more than 1 time in some
hotplug cases,it will cause the kernel oops. This patch checked
dbc_tty_driver to make sure it is unregistered only 1 time.

[  175.741404] BUG: unable to handle kernel NULL pointer dereference at 0000000000000034
[  175.742309] IP: tty_unregister_driver+0x9/0x70
[  175.743148] PGD 0 P4D 0
[  175.743981] Oops: 0000 [#1] SMP PTI
[  175.753904] RIP: 0010:tty_unregister_driver+0x9/0x70
[  175.754817] RSP: 0018:ffffa8ff831d3bb0 EFLAGS: 00010246
[  175.755753] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
[  175.756685] RDX: ffff92089c616000 RSI: ffffe64fe1b26080 RDI: 0000000000000000
[  175.757608] RBP: ffff92086c988230 R08: 000000006c982701 R09: 00000001801e0016
[  175.758533] R10: ffffa8ff831d3b48 R11: ffff92086c982100 R12: ffff92086c98827c
[  175.759462] R13: ffff92086c988398 R14: 0000000000000060 R15: ffff92089c5e9b40
[  175.760401] FS:  0000000000000000(0000) GS:ffff9208a0100000(0000) knlGS:0000000000000000
[  175.761334] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  175.762270] CR2: 0000000000000034 CR3: 000000011800a003 CR4: 00000000003606e0
[  175.763225] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  175.764164] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  175.765091] Call Trace:
[  175.766014]  xhci_dbc_tty_unregister_driver+0x11/0x30
[  175.766960]  xhci_dbc_exit+0x2a/0x40
[  175.767889]  xhci_stop+0x57/0x1c0
[  175.768824]  usb_remove_hcd+0x100/0x250
[  175.769708]  usb_hcd_pci_remove+0x68/0x130
[  175.770574]  pci_device_remove+0x3b/0xc0
[  175.771435]  device_release_driver_internal+0x157/0x230
[  175.772343]  pci_stop_bus_device+0x74/0xa0
[  175.773205]  pci_stop_bus_device+0x2b/0xa0
[  175.774061]  pci_stop_bus_device+0x2b/0xa0
[  175.774907]  pci_stop_bus_device+0x2b/0xa0
[  175.775741]  pci_stop_bus_device+0x2b/0xa0
[  175.776618]  pci_stop_bus_device+0x2b/0xa0
[  175.777452]  pci_stop_bus_device+0x2b/0xa0
[  175.778273]  pci_stop_bus_device+0x2b/0xa0
[  175.779092]  pci_stop_bus_device+0x2b/0xa0
[  175.779908]  pci_stop_bus_device+0x2b/0xa0
[  175.780750]  pci_stop_bus_device+0x2b/0xa0
[  175.781543]  pci_stop_and_remove_bus_device+0xe/0x20
[  175.782338]  pciehp_unconfigure_device+0xb8/0x160
[  175.783128]  pciehp_disable_slot+0x4f/0xd0
[  175.783920]  pciehp_power_thread+0x82/0xa0
[  175.784766]  process_one_work+0x147/0x3c0
[  175.785564]  worker_thread+0x4a/0x440
[  175.786376]  kthread+0xf8/0x130
[  175.787174]  ? rescuer_thread+0x360/0x360
[  175.787964]  ? kthread_associate_blkcg+0x90/0x90
[  175.788798]  ret_from_fork+0x35/0x40

Cc: <stable@vger.kernel.org> # 4.16
Fixes: dfba2174 ("usb: xhci: Add DbC support in xHCI driver")
Signed-off-by: Zhengjun Xing <zhengjun.xing@linux.intel.com>
Tested-by: Christian Kellner <christian@kellner.me>
Reviewed-by: Christian Kellner <christian@kellner.me>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/host/xhci-dbgtty.c

@@ -320,9 +320,11 @@ int xhci_dbc_tty_register_driver(struct xhci_hcd *xhci)
 
 void xhci_dbc_tty_unregister_driver(void)
 {
-	tty_unregister_driver(dbc_tty_driver);
-	put_tty_driver(dbc_tty_driver);
-	dbc_tty_driver = NULL;
+	if (dbc_tty_driver) {
+		tty_unregister_driver(dbc_tty_driver);
+		put_tty_driver(dbc_tty_driver);
+		dbc_tty_driver = NULL;
+	}
 }
 
 static void dbc_rx_push(unsigned long _port)