s2io: Update driver to use dma_wmb

This change updates several spots where a wmb was being used to instead use
a dma_wmb to flush out writes before updating the control portion of the
descriptor.

Signed-off-by: Alexander Duyck <alexander.h.duyck@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>