hwmon: (lm75) added error handling

Add error handling so if lm75_update_device fails
an error is returned when reading the value through sysfs.
This is closely modeled after the way this is handled in ltc4261.

Signed-off-by: Frans Meulenbroeks <fransmeulenbroeks@gmail.com>
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>