V4L/DVB (8642): cx23885: Factor out common cx23885 tuner callback

Tuners currently hook different things to the private pointer in their
callback function. Longer term we should make that private pointer
consistent, but for now separate out the guts of the cx23885 tuner callback
so we can reuse it.

Signed-off-by: Steven Toth <stoth@hauppauge.com>
Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>