net: dsa: don't call switchdev_bridge_port_unoffload for unoffloaded bridge ports

For ports that have a NULL dp->bridge_dev, dsa_port_to_bridge_port()
also returns NULL as expected.

Issue #1 is that we are performing a NULL pointer dereference on brport_dev.

Issue #2 is that these are ports on which switchdev_bridge_port_offload
has not been called, so we should not call switchdev_bridge_port_unoffload
on them either.

Both issues are addressed by checking against a NULL brport_dev in
dsa_port_pre_bridge_leave and exiting early.

Fixes: 2f5dc00f ("net: bridge: switchdev: let drivers inform which bridge ports are offloaded")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>