Use menuconfig objects: ISDN

Unclutter the ISDN menu a tiny bit by moving ISDN4Linux and the CAPI2.0
layers into their own menu.

Signed-off-by: Jan Engelhardt <jengelh@gmx.de>
Cc: Tilman Schmidt <tilman@imap.cc>
Acked-by: Karsten Keil <kkeil@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>