media: vidtv: adapter->mdev was set too late

The media device has to be initialized and assigned to adapter->mdev
before the dvb devices are created, since that will trigger the
automatic creation of the topology.

Rework this code to achieve this.

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