X25: Use identifiers for hdlc x25 device to x25 interface

Change magic numbers to identifiers for X25 interface.

Signed-off-by: Andrew Hendry <andrew.hendry@gmail.com>
Acked-by: Krzysztof Halasa <khc@pm.waw.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>