net: dsa: b53: Export b53_imp_vlan_setup()

bcm_sf2 and b53 do exactly the same thing, so share that piece.

Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>