net: mv643xx_eth: Use dma_map_single() to map the skb fragments

Using dma_map_single() instead of skb_frag_dma_map() allows to unmap
all the descriptors using dma_unmap_single(). This change allows
to introduce software TSO in a less intrusive way.

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