Avoid double call to set ofono's online property

On initialization a double call to set-soft was being done for the ofono device. This caused bugs LP #1392397 and #1379807.
Before this, check that user_data is a valid UrfOfonoDevice object:

g_return_if_fail (URF_IS_OFONO_DEVICE (user_data));

self = URF_OFONO_DEVICE (user_data);
priv = URF_OFONO_DEVICE_GET_PRIVATE (self);

 etc.