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().

Leaving address learning enabled while ports are standalone breaks any
kind of communication which involves port B receiving what port A has
sent. Notably it breaks the ksz9477 driver used with a (non offloaded,
ports act as if standalone) bonding interface in active-backup mode,
when the ports are connected together through external switches, for
redundancy purposes.

This fixes a major design flaw in the ksz9477 and ksz8795 drivers, which
unconditionally leave address learning enabled even while ports operate
as standalone.

Fixes: b987e98e ("dsa: add DSA switch driver for Microchip KSZ9477")
Link: https://lore.kernel.org/netdev/CAFZh4h-JVWt80CrQWkFji7tZJahMfOToUJQgKS5s0_=9zzpvYQ@mail.gmail.com/Reported-by: Brian Hutchinson <b.hutchman@gmail.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20220818164809.3198039-1-vladimir.oltean@nxp.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>