hwmon: (adt7470) Fix writes to temperature limit registers

Temperature limit registers are signed. Limits therefore need
to be clamped to (-128, 127) degrees C and not to (0, 255)
degrees C.

Without this fix, writing a limit of 128 degrees C sets the
actual limit to -128 degrees C.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Cc: stable@vger.kernel.org
Reviewed-by: Axel Lin <axel.lin@ingics.com>