sh_eth: Fix DMA-API usage for RX buffers

- Use the return value of dma_map_single(), rather than calling
  virt_to_page() separately
- Check for mapping failue
- Call dma_unmap_single() rather than dma_sync_single_for_cpu()

Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>