V4L/DVB (10651): uvcvideo: Ignore empty bulk URBs

Devices may send a zero-length packet to signal the end of a bulk payload. If
the payload size is a multiple of the URB size the zero-length packet will be
received by the URB completion handler. Handle this by ignoring all empty URBs.

Signed-off-by: Laurent Pinchart <laurent.pinchart@skynet.be>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>