usbip: Fix vep_free_request() null pointer checks on input args

Fix vep_free_request() to return when usb_ep and usb_request are null
instead of calling WARN_ON.

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