cxgb3/4/4vf: Update drivers to use dma_rmb/wmb where appropriate

Update the Chelsio Ethernet drivers to use the dma_rmb/wmb calls instead of
the full barriers in order to improve performance.

Cc: Santosh Raspatur <santosh@chelsio.com>
Cc: Hariprasad S <hariprasad@chelsio.com>
Cc: Casey Leedom <leedom@chelsio.com>
Signed-off-by: Alexander Duyck <alexander.h.duyck@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>