net: mvpp2: fix missing DMA region unmap in egress processing

The Tx descriptor release code currently calls dma_unmap_single() and
dev_kfree_skb_any() if the descriptor is associated with a non-NULL skb.
This condition is true only for the last fragment of the packet.

Since every descriptor's buffer is DMA-mapped it has to be properly
unmapped.

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>