dpaa2-eth: unmap the SGT buffer before accessing its contents

DMA unmap the Scatter/Gather table before going through the array to
unmap and free each of the header and data chunks. This is so we do not
touch the data between the dma_map and dma_unmap calls.

Fixes: 3dc709e0 ("dpaa2-eth: add support for software TSO")
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>