hwmon: (s3c-hwmon) Ignore invalid divider settings

Avoid registering channels that have zero divider settings in them, as this
will only lead to kernel OOPS from divide-by-zero when the sysfs entry is
read.

Signed-off-by: Ben Dooks <ben@simtec.co.uk>
Signed-off-by: Jean Delvare <khali@linux-fr.org>