hwmon: (nct7802) Fix integer overflow seen when writing voltage limits

Writing a large value into a voltage limit attribute can result
in an overflow due to an auto-conversion from unsigned long to
unsigned int.

Cc: Constantine Shulyupin <const@MakeLinux.com>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Cc: stable@vger.kernel.org # v4.1+
Signed-off-by: Guenter Roeck <linux@roeck-us.net>