V4L/DVB (10268): Proper implement set_voltage in cx24116.

Now there is a card, which uses cx24116 to control LNB DC voltage.
It is DVBWorld DVBS2 PCI-e 2005.
The patch is nedded to add support for that card.
1. Rename CMD_SET_TONEPRE to CMD_LNBDCLEVEL.
2. Fill set_voltage with actually control voltage code.
3. Correct set_tone to not affect voltage.

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