[media] cx24117: Fix LNB set_voltage function

This patch should fix/enhance the set_voltage function for
the cx24117 demodulator.

Signed-off-by: Luis Alves <ljalvs@gmail.com>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>