hwmon: (ds620) Fix overflows seen when writing temperature limits

Module test reports:

temp1_max: Suspected overflow: [160000 vs. 0]
temp1_min: Suspected overflow: [160000 vs. 0]

This is seen because the values passed when writing temperature limits
are unbound.

Reviewed-by: Jean Delvare <jdelvare@suse.de>
Fixes: 60994698 ("hwmon: Support for Dallas Semiconductor DS620")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>