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>

MAINTAINERS

@@ -2197,7 +2197,7 @@ S:	Maintained
 ISDN SUBSYSTEM
 P:	Karsten Keil
 M:	kkeil@suse.de
-L:	isdn4linux@listserv.isdn4linux.de
+L:	isdn4linux@listserv.isdn4linux.de (subscribers-only)
 W:	http://www.isdn4linux.de
 T:	git kernel.org:/pub/scm/linux/kernel/kkeil/isdn-2.6.git
 S:	Maintained
@@ -2205,7 +2205,7 @@ S:	Maintained
 ISDN SUBSYSTEM (Eicon active card driver)
 P:	Armin Schindler
 M:	mac@melware.de
-L:	isdn4linux@listserv.isdn4linux.de
+L:	isdn4linux@listserv.isdn4linux.de (subscribers-only)
 W:	http://www.melware.de
 S:	Maintained