Some cameras will pretty much entirely fill all the image buffers all the
time even though they are using compression.

This patch adds a flag to sd_desc, which drivers for such cameras can set.

When this flag is set the bandwidth calculation code will no longer
assume that the image buffer size is a worst case and less bandwidth than
imagebufsize * fps will be used on average.

This patch sets this new flag for 3 drivers:
* For spca561 (for rev12a cameras) and nw80x cams as these simply don't work
  when given less bandwidth than imagebufsize * fps.
* For sn9c20x cameras, because these show severy jpeg artifacts when
  given less bandwidth than imagebufsize * fps and since these are usb2
  cameras there is plenty bandwidth anyways.
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jean-François Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>