gve: Fixes DMA synchronization.
    
    Synces the DMA buffer properly in order for CPU and device to see
    the most up-to-data data.
    
    Signed-off-by: Yangchun Fu <yangchun@google.com>
    Reviewed-by: Catherine Sullivan <csully@google.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>