usbip: Remove unnecessary null check

"vdev" points to vhci_hcd->vdev[] array and vhci_hcd->vdev[] array
is not a pointer array but a structure array and it is already used
in vhci_urb_enqueue() and then passed to vhci_tx_urb() as an argument.
vhci_tx_urb() is not called except vhci_urb_enqueue(). So, "vdev"
can not be null pointer. This null check statement is meaningless.

Signed-off-by: Suwan Kim <suwan.kim027@gmail.com>
Acked-by: Shuah Khan <shuah@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>