net: dsa: mv88e6xxx: Remove some bureaucracy around querying the VTU

The hardware has a somewhat quirky protocol for reading out the VTU
entry for a particular VID. But there is no reason why we cannot
create a better API for ourselves in the driver.

Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>