[media] media: rc: if protocols can't be changed, don't be writable

If the protocols of an rc device cannot be changed, ensure the sysfs
file is not writable.

This makes it possible to detect this from userspace, so ir-keytable
can deal with case without giving an error.

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