[media] v4l2-of: fix compiler errors if CONFIG_OF is undefined

You must use static inline otherwise you get these errors if CONFIG_OF is not defined:

In file included from drivers/media/platform/soc_camera/soc_camera.c:39:0:
include/media/v4l2-of.h:112:13: warning: 'v4l2_of_free_endpoint' defined but not used [-Wunused-function]
 static void v4l2_of_free_endpoint(struct v4l2_of_endpoint *endpoint)
             ^
In file included from drivers/media/platform/soc_camera/atmel-isi.c:28:0:
include/media/v4l2-of.h:112:13: warning: 'v4l2_of_free_endpoint' defined but not used [-Wunused-function]
 static void v4l2_of_free_endpoint(struct v4l2_of_endpoint *endpoint)
             ^
In file included from drivers/media/platform/soc_camera/rcar_vin.c:36:0:
include/media/v4l2-of.h:112:13: warning: 'v4l2_of_free_endpoint' defined but not used [-Wunused-function]
 static void v4l2_of_free_endpoint(struct v4l2_of_endpoint *endpoint)
             ^

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>