hwmon: (emc2103) Fix overflows seen when temperature limit attributes

Writes into temperature limit attributes can overflow due to unbound
values passed to DIV_ROUND_CLOSEST().

Cc: Steve Glendinning <steve.glendinning@shawell.net>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>