Fix broken CLIR in isdn driver

I noticed that CLIR (aka "hide your calling number") in isdn_tty is broken:
The at-command parser filters out the required "R" (e.g.  ATDR089123456)
It's been broken for a *very* long time.

Signed-off-by: Karsten Keil <kkeil@suse.de>
Signed-off-by: Matthias Goebl <matthias.goebl@goebl.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>