net: dsa: bcm_sf2: Wire-up br_flags_pre, br_flags and set_mrouter

Because bcm_sf2 implements its own dsa_switch_ops we need to export the
b53_br_flags_pre(), b53_br_flags() and b53_set_mrouter so we can wire-up
them up like they used to be with the former b53_br_egress_floods().

Fixes: a8b659e7

 ("net: dsa: act as passthrough for bridge port flags")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>