media: cafe-driver: use drvdata instead of back-casting from v4l2_device

Since commit 95db3a60 ("[media] v4l: Add a media_device pointer
to the v4l2_device structure") the v4l core doesn't insist on using drvdata
itself.  Therefore we can use it ourselves, making things somewhat simpler.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>