[media] gspca: New file autogain_functions.h

This file contains functions for the auto gain.
It must be used with the new control mechanism.

Acked-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>