[media] gspca/sn9c20x: Get rid of scale "magic" numbers

Use macros for the supported scales, instead of using magic numbers
from 0 to 3.

Code become cleaner by using macros for it.

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