[media] rc: split nec protocol into its three variants

Currently we do not know what variant (bit length) of the nec protocol
is used, other than from guessing from the length of the scancode. Now
nec will be handled the same way as the sony protocol or the rc6 protocol;
one variant per bit length.

In the future we might want to expose the rc protocol type to userspace
and we don't want to be introducing this world of pain into userspace
too.

Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>