media: cxusb: implement Medion MD95700 digital / analog coexistence

This patch prepares cxusb driver for supporting the analog part of
Medion 95700 (previously only the digital - DVB - mode was supported).

Specifically, it adds support for:
* switching the device between analog and digital modes of operation,
* enforcing that only one mode is active at the same time due to hardware
limitations.

Actual implementation of the analog mode will be provided by the next
commit.

Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>