X25: Use identifiers for isdn device to x25 interface

Change magic numbers to identifiers for X25 interface.
also minor check patch formatting.

Signed-off-by: Andrew Hendry <andrew.hendry@gmail.com>
Acked-by: Karsten Keil <isdn@linux-pingi.de>
Signed-off-by: David S. Miller <davem@davemloft.net>