V4L/DVB: ds3000: fix divide-by-zero error in ds3000_read_snr()

Fix a divide-by-zero error in ds3000's ds3000_read_snr(), when getting
a very low signal reading (dvbs2_signal_reading >= 1). This prevents
some nasty EIPs when running szap-s2 with a very low signal strength.

Signed-off-by: Nicolas Noirbent <nicolas.noirbent@smartjog.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>