V4L/DVB (7691): pvrusb2-dvb: Don't initialize if device lacks a digital side

In the end we'd like the dvb interface to always be present - even for
analog devices (via the mpeg encoder).  However right now pvrusb2-dvb
won't operate correctly if the hardware doesn't have a digital tuner,
so don't initialize the DVB interface unless we know we have a digital
tuner.

Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>