[media] media/cec.h: use IS_REACHABLE instead of IS_ENABLED

Fix messages like this:

adv7842.c:(.text+0x2edadd): undefined reference to `cec_unregister_adapter'

when CEC_CORE=m but the driver including media/cec.h is built-in. In that case
the static inlines provided in media/cec.h should be used by that driver.

Reported-by: Randy Dunlap <[e-mail]>
Reported-by: kbuild test robot <[e-mail]>
Signed-off-by: Hans Verkuil <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

include/media/cec.h

···
 #define cec_phys_addr_exp(pa) \
 	((pa) >> 12), ((pa) >> 8) & 0xf, ((pa) >> 4) & 0xf, (pa) & 0xf
 
-#if IS_ENABLED(CONFIG_CEC_CORE)
+#if IS_REACHABLE(CONFIG_CEC_CORE)
 struct cec_adapter *cec_allocate_adapter(const struct cec_adap_ops *ops,
 		void *priv, const char *name, u32 caps, u8 available_las);
 int cec_register_adapter(struct cec_adapter *adap, struct device *parent);