drivers/isdn/i4l/isdn_tty.c: fix check for array overindexing

The check for overindexing of dev->mdm.info[] has an off-by-one.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Karsten Keil <keil@b1-systems.de>