hwmon: (twl4030-madc-hwmon) Return proper error if hwmon_device_register fails

The driver did not return an error if the call to hwmon_device_register failed.
Fix by returning the error reported from hwmon_device_register.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>