rocker: handle non-bridge master change

Master change notifications may occur other than when joining or
leaving a bridge, for example when being added to or removed from
a bond or Open vSwitch.

Previously in those cases rocker_port_bridge_leave() was called
which results in a null-pointer dereference as rocker_port->bridge_dev
is NULL because there is no bridge device.

This patch makes provision for doing nothing in such cases.

Fixes: 6c707945

 ("rocker: implement L2 bridge offloading")
Acked-by: Jiri Pirko <jiri@resnulli.us>
Acked-by: Scott Feldman <sfeldma@gmail.com>
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>