hwmon: (ina3221) Validate shunt resistor value from DT

The input->shunt_resistor is int type while the value from DT is
unsigned int. Meanwhile, a divide-by-zero error would happen if
the value is 0. So this patch just simply validates the value.

Signed-off-by: Nicolin Chen <nicoleotsuka@gmail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>