ax25: Kconfig: Update link for linux-ax25.org

http://linux-ax25.org has been down for nearly a year. Its official
replacement is https://linux-ax25.in-berlin.de. Change all references to
the old site in the ax25 Kconfig to its replacement.

Link: https://marc.info/?m=166792551600315


Signed-off-by: Peter Lafreniere <peter@n8pjl.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>