hwmon: (lm73) Fix lower and upper temperature limits

While the LM73 is only specified for temperatures from -40 to +150 degrees C,
its power-up minimum and maximum temperature limits are -256 and +255.75
degrees C. For better consistency and to avoid confusion, clamp limits to
the power-up limits and not to -40 / +150 degrees C.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>