V4L/DVB (7451): cx88: Add user control for chroma AGC

The cx2388x family has support for chroma AGC. This patch implements a
the V4L2_CID_CHROMA_AGC control for the cx2388x family. By default
chroma AGC is disabled, as in previous versions of the driver.

Signed-off-by: "Frej Drejhammar <frej.drejhammar@gmail.com&gt;">
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>