ISDN4Linux: fix maturity label

According to the definitions recently posted on LKML, the maturity label
for the ISDN4Linux subsystem is wrong.  This patch corrects it and also
clarifies the accompanying help text a bit.

Signed-off-by: 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>