media: gspca: Limit frame size to sizeimage.

Limit frame size to what userland code expects. This can happen when
cameras, such as Kensington VideoCAM, use fixed sized transfer packets
which includes trailing junk in the final packet.

Signed-off-by: Chad Fraleigh <chadf@triularity.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>