WAN: HDLC: Call notifiers before and after changing device type

An HDLC device can change type when the protocol driver is changed.
Calling the notifier change allows potential users of the interface
know about this planned change, and even block it. After the change
has occurred, send a second notification to users can evaluate the new
device type etc.

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