V4L/DVB (4284): Cx24123: fix set_voltage function according to the specs

The set_voltage function in cx24123.c was corrected to match how it is
described in the CX24123 specs, producing the correct behaviour for cards
that require it.
Acked-by: Andrew de Quincey <adq_dvb@lidskialf.net>

Signed-off-by: Saqeb Akhter <johoja@gmail.com>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>