net: dsa: b53: Export b53_configure_vlan()

bcm_sf2 and b53 replicate the same operations: clear all VLANs and set
their ports to the default VLAN tag (1 for these devices) so export the
b53 function doing just that.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>