[media] v4l2: uvcvideo: Allow using larger buffers

A test in uvc_video_decode_isoc() checks whether an image has been
received from the camera completely. For this the data amount is compared
to the buffer length, which, however, doesn't have to be equal to the
image size. Switch to using formats .sizeimage field for an exact
expected image size.

[Renamed image_size to frame_size]

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>