hwmon: (w83791d) Drop unnecessary compare statements

The following build warnings are seen with -Wextra.

w83791d.c: In function store_temp_target:
w83791d.c:858:2: warning: comparison of unsigned expression < 0 is always false
w83791d.c: In function store_temp_tolerance:
w83791d.c:920:2: warning: comparison of unsigned expression < 0 is always false

For store_temp_target, accept negative numbers and clamp to >= 0.
For store_temp_tolerance, drop the unnecessary comparison.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Jean Delvare <khali@linux-fr.org>