net: dsa: loop: Return VLAN table size through devlink

We return the VLAN table size through devlink as a simple parameter, we
do not support altering it at runtime:

devlink resource show mdio_bus/fixed-0:1f
mdio_bus/fixed-0:1f:
  name VTU size 4096 occ 0 unit entry dpipe_tables none

and after configure a bridge with VLAN filtering:

devlink resource show mdio_bus/fixed-0:1f
mdio_bus/fixed-0:1f:
  name VTU size 4096 occ 1 unit entry dpipe_tables none

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>