[media] uvcvideo: Fix frame drop in bulk video stream

When video endpoint is configured as bulk, a ZLP is sent after every
video frames with size as multiple of 512 bytes. This is done so that
host can detect end of transfer and pass data for processing.
Still, frames that are multiple of 16K in size gets dropped. The ZLP
sent by camera is ignored by uvc_video_decode_bulk(). The makes sure
that the ZLP is not part of a video frame before ignoring it. If ZLP
follows a video frame, then it triggers completion callback.

[mchehab@redhat.com: Fix a small CodingStyle issue]
Signed-off-by: Jayakrishnan Memana <jayakrishnan.memana@maxim-ic.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>