dpaa2-eth: rearrange variable declaration in __dpaa2_eth_tx

In the next patches we'll be moving things arroung in the mentioned
function and also add some new variable declarations. Before all this,
cleanup the variable declaration order.

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