[media] cx23855: add frontend set voltage function into state

Setting the LNB voltage requires setting some GPIOs on the cx23885 with some boards before calling the actual set_voltage function in the demod driver. Add a function pointer into state for that case.

Signed-off-by: Olli Salonen <olli.salonen@iki.fi>
Acked-by: Antti Palosaari <crope@iki.fi>
Reviewed-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>