hwmon: (emcw201) Fix overflows seen when writing into limit attributes

Writes into temperature and voltage limit attributes can overflow
due to multiplications with unchecked parameters. Also, the input
parameter to DIV_ROUND_CLOSEST() needis to be range checked.

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