media: dvb-usb-v2: add probe/disconnect callbacks

Add probe and disconnect callbacks that behaves similarly than ones
used commonly on Linux driver model. We need those to get early / late
access to driver in order to use normal probe time stuff, like regmap,
extra bus adapters and so.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>