[media] cec: fix confusing CEC_CAP_RC and IS_REACHABLE(CONFIG_RC_CORE) code

It is a bit confusing how CEC_CAP_RC and IS_REACHABLE(CONFIG_RC_CORE)
interact. By stripping CEC_CAP_RC at the beginning rather than after #else
it should be a bit clearer what is going on.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reported-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>