[media] v4l: Forbid usage of V4L2_MBUS_FMT definitions inside the kernel

Place v4l2_mbus_pixelcode in a #ifndef __KERNEL__ section so that kernel
users don't have access to these definitions.

We have to keep this definition for user-space users even though they're
encouraged to move to the new media_bus_format enum.

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