hwmon: (w83793) Clean up a signedness issue

We cap the upper bound of "mtimeout" but since it's signed we should
check for negative values as well.  The mistake is harmless.  But I have
changed it to unsigned as a cleanup.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>