V4L/DVB (7454): cx88: Add user control for color killer

The cx2388x family has a color killer. This patch implements the
V4L2_CID_COLOR_KILLER control for the cx2388x family. By default the
color killer 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>