V4L/DVB (4874): Fix oops on symbol rate==0

The tda10086 causes an oops (divide by zero) if a zero symbol rate is used;
this prevents this.

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