dpaa2-eth: return num_enqueued frames from enqueue callback

The enqueue dpaa2-eth callback now returns the number of successfully
enqueued frames. This is a preliminary patch necessary for adding
support for bulk ring mode enqueue.

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