Fixed to check whether Ethernet cable state changed callback exists when it is released

Fixed to check whether Ethernet cable state changed callback exists when it is released

The callback can be registered for each connection handle
and the ref count is incremented each time it is registered.
However, when the callback is released,
the ref count is decremented without checking whether or not the callback is registered in the connection handle.
Therefore, the ref count can be 0 even though there is a valid callback, and the event may not be processed.

Change-Id: I0569e8d0a74ca647c5bcdba6c877e7374a89e6f9