usb: xhci: fix panic in xhci_free_virt_devices_depth_first

usb: xhci: fix panic in xhci_free_virt_devices_depth_first

Check vdev->real_port 0 to avoid panic
[    9.261347] [<ffffff800884a390>] xhci_free_virt_devices_depth_first+0x58/0x108
[    9.261352] [<ffffff800884a814>] xhci_mem_cleanup+0x1bc/0x570
[    9.261355] [<ffffff8008842de8>] xhci_stop+0x140/0x1c8
[    9.261365] [<ffffff80087ed304>] usb_remove_hcd+0xfc/0x1d0
[    9.261369] [<ffffff80088551c4>] xhci_plat_remove+0x6c/0xa8
[    9.261377] [<ffffff80086e928c>] platform_drv_remove+0x2c/0x70
[    9.261384] [<ffffff80086e6ea0>] __device_release_driver+0x80/0x108
[    9.261387] [<ffffff80086e7a1c>] device_release_driver+0x2c/0x40
[    9.261392] [<ffffff80086e5f28>] bus_remove_device+0xe0/0x120
[    9.261396] [<ffffff80086e2e34>] device_del+0x114/0x210
[    9.261399] [<ffffff80086e9e00>] platform_device_del+0x30/0xa0
[    9.261403] [<ffffff8008810bdc>] dwc3_otg_work+0x204/0x488
[    9.261407] [<ffffff80088133fc>] event_work+0x304/0x5b8
[    9.261414] [<ffffff80080e31b0>] process_one_work+0x148/0x490
[    9.261417] [<ffffff80080e3548>] worker_thread+0x50/0x4a0
[    9.261421] [<ffffff80080e9ea0>] kthread+0xe8/0x100
[    9.261427] [<ffffff8008083680>] ret_from_fork+0x10/0x50

The problem can occur if xhci_plat_remove() is called shortly after
xhci_plat_probe(). While xhci_free_virt_devices_depth_first been
called before the device has been setup and get real_port initialized.
The problem occurred on Hikey960 and was reproduced by Guenter Roeck
on Kevin with chromeos-4.4.

Fixes: ee8665e28e8d ("xhci: free xhci virtual devices with leaf nodes first")
Cc: Guenter Roeck <groeck@google.com>
Cc: <stable@vger.kernel.org> # v4.10+
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Fan Ning <fanning4@hisilicon.com>
Signed-off-by: Li Rui <lirui39@hisilicon.com>
Signed-off-by: yangdi <yangdi10@hisilicon.com>
Signed-off-by: Yu Chen <chenyu56@huawei.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>