[WAN]: Added missing netif_dormant_off() to generic HDLC

WAN: Fixed a problem with PPP/raw HDLC/X.25 protocols not doing
netif_dormant_off() at startup.

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