net: pxa168_eth: Use dma_wmb/rmb where appropriate

Update the pxa168_eth driver to use the dma_rmb/wmb calls instead of the
full barriers in order to improve performance: reduced 97ns/39ns on
average in tx/rx path on Marvell BG4CT platform.

Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>