V4L/DVB (9048): add a general-purpose callback pointer to struct dvb_frontend

Remove tuner_callback pointers from tuner driver configuration
and private state structures, replaced with a general-purpose
callback pointer within struct dvb_frontend.

A new parameter is added to the callback function, called
component. This allows us to use this callback pointer by
frontend components other than the tuner, if need be. So
far, this is only used by tuner drivers.

Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>