[media] gspca - main: Use a better altsetting for image transfer

Previously image transfer used to use the highest altsetting, which, most of
the time, took the whole USB bandwidth.
Now, the altsetting is chosen according to an estimation of the bandwidth
needed for image transfer.

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