drivers/cc110x: Adapt cc110x netdev impl to changed rcv event handling

This PR adapts the CC110X netdev implementation to the updated receive event handling sequence for netdev (see [description](https://github.com/RIOT-OS/RIOT/wiki/Model-for-the-network-stack#receive-packet-from-hardware)).

On packet reception, in the interrupt routine the packet is read from the CC110X and stored in the device driver's RX-Buffer. The event handler thread is notified of the receive event with a msg and the interrupt routine is left. The event handler thread then calls `_cc110x_event` with the fitting event type. In the event function the packet is read from the device driver's RX-Buffer and the registered callback is executed. This sequence is needed to make sure that the callback execution is taking place in the event handler thread.

The PR is depending on #2163.
I haven't really understood yet what the benefits of having multiple netdev "instances" are? Could you explain this with a proper use-case?