[WAN]: converting generic HDLC to use netif_dormant*()

This patch converts generic HDLC (and WAN drivers using it) from
hdlc_set_carrier() to netif_dormant*() interface.
WAN hardware drivers should now use netif_carrier_on|off() like
other network drivers.

Signed-off-by: Krzysztof Halasa <khc@pm.waw.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>