[media] gspca - main: Better values for V4L2_FMT_FLAG_COMPRESSED

In vidioc_enum_fmt_vid_cap, the 'compressed' flag was set from the
pixelformat, and all pixelformats were not treated.
Now, images are told as compressed when the image size is smaller than
its number of pixels.

Signed-off-by: Jean-François Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>