[media] bt8xx: Add video4linux control V4L2_CID_COLOR_KILLER

Added V4L2_CID_COLOR_KILLER control to the bt8xx driver.
The control V4L2_CID_PRIVATE_CHROMA_AGC was changed too because
with this change the bttv driver must touch two bits in the
SC Loop Control Registers, for controls V4L2_CID_COLOR_KILLER
and V4L2_CID_PRIVATE_CHROMA_AGC.

Signed-off-by: Guilherme Herrmann Destefani <linuxtv@destefani.eng.br>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>