b43legacy: fix DMA mapping leakage

This fixes a DMA mapping leakage in the case where we reject a DMA buffer
because of its address.
The patch by Michael Buesch has been ported to b43legacy.

Signed-off-by: Stefano Brivio <stefano.brivio@polimi.it>
Cc: Christian Casteyde <casteyde.christian@free.fr>
Signed-off-by: John W. Linville <linville@tuxdriver.com>