Use menuconfig objects: ISDN: CONFIG_ISDN_CAPI

Transform "depends on" into a simpler if-endif block style dependency.

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