V4L/DVB (13838): atbm8830: use AGC setting from config

Improves ATBM8830 reception by using per card AGC configuration rather
than register default.

Signed-off-by: David T. L. Wong <davidtlwong@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>