media: rc: ensure we do not read out of bounds

If rc_validate_filter() is called for CEC or XMP, then we would read
beyond the end of the array.

Suggested-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>