hwmon: (adm1026) Fix overflows seen when writing into limit attributes

Fix overflows seen when writing large values into voltage limit,
temperature limit, temperature offset, and DAC attributes.

Overflows are seen due to unbound multiplications and additions.

While at it, change the low temperature limit to -128 degrees C,
since this is the minimum temperature accepted by the chip.

Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>