b43: Fix DMA mapping leakage

This fixes a DMA mapping leakage in the case where we reject a DMA
buffer because of its address.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Cc: Christian Casteyde <casteyde.christian@free.fr>
Signed-off-by: John W. Linville <linville@tuxdriver.com>