isdn: avoid copying overly-long strings

Addresses http://bugzilla.kernel.org/show_bug.cgi?id=9416

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