[media] cec: improve MEDIA_CEC_RC dependencies

[media] cec: improve MEDIA_CEC_RC dependencies

Changing the IS_REACHABLE() into a plain #ifdef broke the case of
CONFIG_MEDIA_RC=m && CONFIG_MEDIA_CEC=y:

drivers/media/cec/cec-core.o: In function `cec_unregister_adapter':
cec-core.c:(.text.cec_unregister_adapter+0x18): undefined reference to `rc_unregister_device'
drivers/media/cec/cec-core.o: In function `cec_delete_adapter':
cec-core.c:(.text.cec_delete_adapter+0x54): undefined reference to `rc_free_device'
drivers/media/cec/cec-core.o: In function `cec_register_adapter':
cec-core.c:(.text.cec_register_adapter+0x94): undefined reference to `rc_register_device'
cec-core.c:(.text.cec_register_adapter+0xa4): undefined reference to `rc_free_device'
cec-core.c:(.text.cec_register_adapter+0x110): undefined reference to `rc_unregister_device'
drivers/media/cec/cec-core.o: In function `cec_allocate_adapter':
cec-core.c:(.text.cec_allocate_adapter+0x234): undefined reference to `rc_allocate_device'
drivers/media/cec/cec-adap.o: In function `cec_received_msg':
cec-adap.c:(.text.cec_received_msg+0x734): undefined reference to `rc_keydown'
cec-adap.c:(.text.cec_received_msg+0x768): undefined reference to `rc_keyup'

This adds an additional dependency to explicitly forbid this combination.

Fixes: 5f2c467c54f5 ("[media] cec: add MEDIA_CEC_RC config option")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>