dpaa2-switch: use the port index in the IRQ handler

The MC firmware supplies us the switch interface index for which an
interrupt was triggered. Use this to our advantage instead of looping
through all the switch ports and doing unnecessary work.

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