V4L/DVB (13480): atbm8830: fix compile warning

Fix this trivial compile warning:

v4l/atbm8830.c:164: warning: 'set_agc_config' defined but not used

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>