V4L/DVB (9051): dib0700: use dvb_frontend->callback for xc5000 gpio reset

The tuner_callback function pointer from struct xc5000_config has been removed.
Use dvb_frontend->callback instead.

Also, mark function dib0700_xc5000_tuner_callback as static int.

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