WAN: fix Cisco HDLC handshaking.

Cisco HDLC uses keepalive packets and sequence numbers to determine link
state. In rare cases both ends could transmit keepalive packets at the same
time, causing the received sequence numbers to be treated as incorrect.
Now we accept our current sequence number as well as the previous one.

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