rocker: Remove port_attr_bridge_flags_get assignment

After 610d2b60 ("rocker: Remove getting PORT_BRIDGE_FLAGS") we no
longer have a port_attr_bridge_flags_get member in the rocker_world_ops
structre, fix that.

Fixes: 610d2b60

 ("rocker: Remove getting PORT_BRIDGE_FLAGS")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>