net: dsa: b53: Report VLAN table occupancy via devlink

We already maintain an array of VLANs used by the switch so we can
simply iterate over it to report the occupancy via devlink.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>