MAINTAINERS: isdn4linux@listserv.isdn4linux.de is subscribers-only

    
https://www.isdn4linux.de/mailman/listinfo/isdn4linux: "To prevent spamming,
you have to subscribe first. Mails from non-members are silently ignored!"
    
Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: David S. Miller <davem@davemloft.net>