b43legacy: Fix usage of struct device used for DMAing

This fixes b43legacy for the SSB DMA API change.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Cc: Stefano Brivio <stefano.brivio@polimi.it>
Signed-off-by: John W. Linville <linville@tuxdriver.com>