net: dsa: mv88e6xxx: change default return of mv88e6xxx_port_bridge_flags

The default return value -EOPNOTSUPP of mv88e6xxx_port_bridge_flags()
came from the return value of the DSA method port_egress_floods() in
commit 4f85901f ("net: dsa: mv88e6xxx: add support for bridge flags"),
but the DSA API was changed in commit a8b659e7 ("net: dsa: act as
passthrough for bridge port flags"), resulting in the return value
-EOPNOTSUPP not being valid anymore, and sections for new flags will not
need to set the return value to zero on success, as with the new mab flag
added in a following patch.

Signed-off-by: Hans J. Schultz <netdev@kapio-technology.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>