media: staging/imx: Don't set driver data for v4l2_dev

The media device is already available via multiple methods, there is no
need to set driver data for v4l2_dev to the media device.

In imx_media_link_notify(), get media device from link->graph_obj.mdev.

In imx_media_capture_device_register(), get media device from
v4l2_dev->mdev.

Signed-off-by: Steve Longerbeam <slongerbeam@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>