linuxdvb: fix diseqc switch control (thanks to amet for fixing/testing).

The problem was that I played fast and loose with the diseqc spec. I set the
line volts much later than you're supposed to, for various reasons.

I've now sorted that, though because of the complexity if a rotor exists I
run a full re-program of the switch (only if a rotor is present). And I double
set the line volts, both in the switch (to ensure it happens early) and then
in the LNB tuning (in case no switch exists).