media: pvrusb2: Add i2c client demod/tuner support

i2c client device is the "new" method to attach to dvb modules, include
support for this functionality. Cleanup code has been added to init in
case of failure, as well as to frontend exit.

Required by Hauppauge HVR-1975

Signed-off-by: Brad Love <brad@nextdimension.cc>
Reviewed-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>