net: systemport: Add support for VLAN transmit acceleration

SYSTEMPORT is capable of performing VLAN transmit acceleration, support
that by configuring it appropriately, providing the VLAN ID and PCP/DEI
where necessary.

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