mlxsw: spectrum: Handle PORT_PRE_BRIDGE_FLAGS

In preparation for getting rid of switchdev_port_attr_get(), have mlxsw
check for the bridge flags being set through switchdev_port_attr_set()
when the SWITCHDEV_ATTR_ID_PORT_PRE_BRIDGE_FLAGS attribute identifier is
used.

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