net: marvell: prestera: fix brige port operation

Return NOTIFY_DONE (dont't care) for switchdev notifications
that prestera driver don't know how to handle them.

With introduction of SWITCHDEV_BRPORT_[UN]OFFLOADED switchdev
events, the driver rejects adding swport to bridge operation
which is handled by prestera_bridge_port_join() func. The root
cause of this is that prestera driver returns error (EOPNOTSUPP)
in prestera_switchdev_blk_event() handler for unknown swdev
events. This causes switchdev_bridge_port_offload() to fail
when adding port to bridge in prestera_bridge_port_join().

Fixes: 957e2235 ("net: make switchdev_bridge_port_{,unoffload} loosely coupled with the bridge")
Signed-off-by: Volodymyr Mytnyk <vmytnyk@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>