dpaa2-eth: Poll Tx pending frames counter on if down

Starting with firmware version MC10.18.0, a new counter for in flight
Tx frames is offered. Use it when bringing down the interface to
determine when all pending Tx frames have been processed by hardware
instead of sleeping a fixed amount of time.

Signed-off-by: Ioana Radulescu <ruxandra.radulescu@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>