dpaa2-switch: setup learning state on STP state change

Depending on what STP state a port is in, the learning on that port
should be enabled or disabled.

When the STP state is DISABLED, BLOCKING or LISTENING no learning should
be happening irrespective of what the bridge previously requested. The
learning state is changed to be the one setup by the bridge when the STP
state is LEARNING or FORWARDING.

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