usbip: Fix vhci_urb_enqueue() URB null transfer buffer error path

commit 2c904963

 upstream.

Fix vhci_urb_enqueue() to print debug msg and return error instead of
failing with BUG_ON.

Signed-off-by: Shuah Khan <shuah@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>