V4L/DVB (9535): cx88-dvb: Remove usage core->prev_set_voltage from cx24116 based cards.

Remove usage core->prev_set_voltage from cx24116 based cards,
as it does not make sense for the demod.

Signed-off-by: Igor M. Liplianin <liplianin@me.by>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>