net: mv643xx_eth: Ensure proper data alignment in TSO TX path

The TX DMA engine requires that buffers with a size of 8 bytes or smaller
must be 64 bit aligned. This requirement may be violated when doing TSO,
as in this case larger skb frags can be broken up and transmitted in small
parts with then inappropriate alignment.

Fix this by checking for proper alignment before handing a buffer to the
DMA engine. If the data is misaligned realign it by copying it into the
TSO header data area.

Signed-off-by: Philipp Kirchhofer <philipp@familie-kirchhofer.de>
Signed-off-by: David S. Miller <davem@davemloft.net>