V4L/DVB: cx88: Let the user disable IR support

It might be useful to be able to disable the IR support, either for
debugging purposes, or just for users who know they won't use the IR
remote control anyway. On many cards, IR support requires expensive
polling/sampling which is better avoided if never needed.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>