ravb: do not invalidate cache for RX buffer twice

First, dma_sync_single_for_cpu() shouldn't have been called in the first place
(it's a streaming DMA API), dma_unmap_single() should have been called instead.
Second, dma_unmap_single() call after handing the buffer to napi_gro_receive()
makes little sense.  Moreover desc->dptr might not be valid at this point.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>