V4L/DVB (6128): hybrid tuner refactoring core changes, phase 1

Prepare tuner-core for conversion of tuner sub-drivers into 
dvb_frontend modules

Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Acked-by: Hans Verkuil <hverkuil@xs4all.nl>
Acked-by: Mike Isely <isely@pobox.com>
Acked-by: Steven Toth <stoth@hauppauge.com>
Acked-by: Patrick Boettcher <pb@linuxtv.org>
Acked-by: Jarod Wilson <jwilson@redhat.com>
Acked-by: Oliver Endriss <o.endriss@gmx.de>
Acked-by: Trent Piepho <xyzzy@speakeasy.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>