net: dsa: b53: implement DSA port fast ageing

Remove the fast ageing logic from b53_br_set_stp_state and implement the
new DSA switch port_fast_age operation instead.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>