V4L/DVB (7679): pvrusb2: add DVB API framework

Add basic framework for the DVB API.  This is enough to control the
tuner & demod of the digital frontend, but the stream & buffer handling
is still missing.

Additional note from Mike Isely <isely@pobox.com> - also, since these
changes are still very experimental arrange for DVB changes to be
compiled in via new CONFIG_VIDEO_PVRUSB2_DVB option, for now.

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