net: moxa: synchronize DMA memory

DMA memory should be synchronized before data is passed
to/from controller.

Add dma_sync_single_for_cpu(.., DMA_FROM_DEVICE) to RX path
and dma_sync_single_for_device(.., DMA_TO_DEVICE) to TX path.

Signed-off-by: Jonas Jensen <jonas.jensen@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>