net: mvpp2: fix buffers' DMA handling on RX path

Each allocated buffer, whose pointer is put into BM pool is DMA-mapped.
Hence it should be properly unmapped after usage or when removing buffers
from pool.

This commit fixes DMA handling on RX path by adding dma_unmap_single() in
mvpp2_rx() and in mvpp2_bufs_free(). The latter function's argument number
had to be increased for this purpose.

Signed-off-by: Marcin Wojtas <mw@semihalf.com>

Fixes: 3f518509 ("ethernet: Add new driver for Marvell Armada 375
network unit")

Cc: <stable@vger.kernel.org> # v3.18+
Signed-off-by: David S. Miller <davem@davemloft.net>