isdn: fix random hard freeze with AVM T1 cards

This fixes the hard freeze debugged for AVM C4 cards for the AVM T1 cards.

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>