hwmon: Allow writing of negative trigger temperatures

- replace differing temperature variable types by long
- use strtol() instead of strtoul() for conversion

Signed-off-by: Christian Hohnstaedt <chohnstaedt@innominate.com>
Acked-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Mark M. Hoffman <mhoffman@lightlink.com>