[NET] X.25 async net_device fixup

[NET] X.25 async net_device fixup

Convert X.25 async driver to have dynamic net_device's.
This driver is a lot like SLIP so the code changes were similar.
        - Added similar locking to SLIP
        - replaced code that snooped for MTU changes with the
          net_device change mtu callback.
        - cleaned up the statistics by using the net_device_stats structure.

Patch is against 2.6.0-test2.

Not sure if anyone ever uses this.  I tested by bringing up an x.25 async
line using a modified version of slattach.