b43: Add RX side DMA memory barrier

This adds a memory barrier to ensure the writes to the ring memory
are committed before the DMA ring pointer is updated.
We do a similar thing on the TX side already.
Signed-off-by: Michael Buesch <m@bues.ch>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/b43/dma.c

@@ -1600,6 +1600,7 @@ void b43_dma_rx(struct b43_dmaring *ring)
 		dma_rx(ring, &slot);
 		update_max_used_slots(ring, ++used_slots);
 	}
+	wmb();
 	ops->set_current_rxslot(ring, slot);
 	ring->current_slot = slot;
 }