net: mvneta: Fix missing DMA region unmap

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 is true only for the last fragment of the packet.

This is wrong, however, since every descriptor buffer is DMA mapped and needs
to be unmapped.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>