media: pvrusb2: Add multiple dvb frontend support

All changes are equivalent and backwards compatible.
All current devices have been changed to use fe[0]
Code has been added to dvb init to support cleanup after failure.

Multiple frontends are required by Hauppauge HVR-1975, which is in a
later commit.

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>