[media] em28xx: introduce #defines for the image quality default settings

The image quality default values will be used in at least two different places
and by using #defines we make sure that they are always consistent.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>