[libqofono] Emit property change notifications signals when setting modemPath

Setting the modemPath causes the local object to connect to a different
remote object. The local cached value of the remote object's properties
were being updated correctly, however, the property change notification
signals were not being emitted preventing QML bindings using those
properties from being reevaluated.

Removed redundant conditional tests either because they were unneeded
or because they had only one possible outcome.