b43legacy: remove b43legacy_dma_set_mask

These days drivers are not required to fallback to smaller DMA masks,
but can just set the largest mask they support, removing the need for
this trial and error logic.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>