media: lirc: ensure RC_PROTO_MAX has documentation

The enum rc_proto value RC_PROTO_MAX has no documentation, this is causing
a warning while building the documentation.

Fixes: 72e637fe ("media: rc: validate that "rc_proto" is reasonable")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>