dpaa2-eth: check the result of skb_to_sgvec()

Before passing the result of skb_to_sgvec() to dma_map_sg() check if any
error was returned.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>