usb: usbip: Remove unnecessary get_vdev()

vhci_tx_urb() should be able to get the vhci_device from
its caller vhci_urb_enqueue(), instead of brutal-force
searching it.

Reviewed-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Signed-off-by: Yuyang Du <yuyang.du@intel.com>
Acked-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>