[media] em28xx: separate sensor detection and initialization/configuration

Sensor detection and initialization/configuration are currently mixed together.
This works as long as all devices with a particular sensor are working with the
same board configuration. In the long run, this will be not sufficient, so
separate these both steps to make the code more flexible and future proof.
This also makes the code more consistent, because the initialization of the
MT9V011 sensor subdevice is already separated.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>