media: rc: if kernel is built without an IR codec, don't advertise it

If the kernel is built without e.g. the imon IR decoder, the protocols
sysfs file still show the protocol as available. If user-space tries to
enable this decoder, the kernel will report an error:

$ echo +imon > /sys/class/rc/rc0/protocols
[   57.693033] rc_core: Loaded IR protocol module ir-imon-decoder, but protocol imon still not available

Ensuring that unavailable protocols are not advertised, ensures that
users space knows it is not available, and a BPF based IR decoder can be
loaded instead.

This supports the case when no kernel-based codec needs to be compiled in,
and every IR decoder can be BPF based.

Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>