V4L/DVB (4831): Fix tuning on older budget DVBS cards.

Fixes to DISEQC on these cards inadvertently broke normal tone/voltage
signalling. This restores the necessary function.

Signed-off-by: Andrew de Quincey <adq_dvb@lidskialf.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>