Address learning should initially be turned off by the driver for port
operation in standalone mode, then the DSA core handles changes to it
via ds->ops->port_bridge_flags().
Currently this is not the case for qca8k where learning is enabled
unconditionally in qca8k_setup for every user port.
Handle ports configured in standalone mode by making the learning
configurable and not enabling it by default.
Implement .port_pre_bridge_flags and .port_bridge_flags dsa ops to
enable learning for bridge that request it and tweak
.port_stp_state_set to correctly disable learning when port is
configured in standalone mode.
Signed-off-by: Christian Marangi <ansuelsmth@xxxxxxxxx>