WAN: HDLC: Detach protocol before unregistering device

The current code first unregisters the device, and then detaches the
protocol from it. This should be performed the other way around, since
the detach may try to use state which has been freed by the
unregister. Swap the order, so that we first detach and then remove the
netdev.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>