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

Add a new module parameter "disable_ir" to disable IR support. Several
other drivers do that already, and this can be very handy for
debugging purposes.

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