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>

include/uapi/linux/lirc.h

@@ -196,6 +196,7 @@ struct lirc_scancode {
  * @RC_PROTO_RCMM24: RC-MM protocol 24 bits
  * @RC_PROTO_RCMM32: RC-MM protocol 32 bits
  * @RC_PROTO_XBOX_DVD: Xbox DVD Movie Playback Kit protocol
+ * @RC_PROTO_MAX: Maximum value of enum rc_proto
  */
 enum rc_proto {
 	RC_PROTO_UNKNOWN	= 0,