net: dsa: loop: PVID should be per-port

The PVID should be per-port, this is a preliminary change to support a
802.1Q data path in the driver.

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>