media: v4l2-device.h: always expose mdev

The mdev field is only present if CONFIG_MEDIA_CONTROLLER is set.
But since we will need to pass the media_device to vb2 and the
control framework it is very convenient to just make this field
available all the time. If CONFIG_MEDIA_CONTROLLER is not set,
then it will just be NULL.

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