V4L/DVB: dvb/dib8000: fix build warning

In file included from drivers/media/dvb/dvb-usb/dib0700_devices.c:14:
  drivers/media/dvb/frontends/dib8000.h: In function 'dib8000_get_adc_power':
  drivers/media/dvb/frontends/dib8000.h:112: warning: no return statement in function returning non-void

Fixed by adding a return to the dummy function.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>