dpaa2-eth: Support multiple traffic classes on Tx

DPNI objects can have multiple traffic classes, as reflected by
the num_tc attribute. Until now we ignored its value and only
used traffic class 0.

This patch adds support for multiple Tx traffic classes; we have
num_queues x num_tcs hardware queues available for each interface.

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