mlxsw: spectrum_switchdev: Remove getting PORT_BRIDGE_FLAGS

There is no code that will query the SWITCHDEV_ATTR_ID_PORT_BRIDGE_FLAGS
attribute remove support for that.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>