hwmon: (adt7470) Prevent divide by zero in adt7470_fan_write()

The "val" variable is controlled by the user and comes from
hwmon_attr_store().  The FAN_RPM_TO_PERIOD() macro divides by "val"
so a zero will crash the system.  Check for that and return -EINVAL.
Negatives are also invalid so return -EINVAL for those too.

Fixes: fc958a61

 ("hwmon: (adt7470) Convert to devm_hwmon_device_register_with_info API")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>