b43legacy: fix DMA for 30/32-bit DMA engines

This checks if the DMA address is bigger than what the controller can manage.
It will reallocate the buffers in the GFP_DMA zone in that case.

The patch by Michael Buesch has been ported to b43legacy.
Thanks to Matti Viljanen for reporting this.

Cc: Matti Viljanen <viljanen.matti@gmail.com>
Signed-off-by: Stefano Brivio <stefano.brivio@polimi.it>
Signed-off-by: John W. Linville <linville@tuxdriver.com>