isdn: fix random hard freeze with AVM cards using b1dma

This fixes the hard freeze debugded for AVM C4 cards using the b1dma
interface.

[akpm@linux-foundation.org: coding-style fixes]
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>