[media] gspca/autogain_functions.h: Allow users to declare what they want

Allow users of gspca/autogain_functions.h to declare which of the autogain
algoritms they are going to use. This allows us to remove the hacks from
drivers which don't use coarse_grained_expo_autogain.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>