[media] cec: Fix runtime BUG when (CONFIG_RC_CORE && !CEC_CAP_RC)

Currently when the RC Core is enabled (reachable) core code located
in cec_register_adapter() attempts to populate the RC structure with
a pointer to the 'parent' passed in by the caller.

Unfortunately if the caller did not specify RC capability when calling
cec_allocate_adapter(), then there will be no RC structure to populate.

This causes a "NULL pointer dereference" error.

Fixes: f51e8080 ("[media] cec: pass parent device in register(), not allocate()")

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Cc: <stable@vger.kernel.org>      # for v4.10 and up
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>