net: dsa: mv88e6xxx: read then write PVID

The port register 0x07 contains more options than just the default VID,
even though they are not used yet. So prefer a read then write operation
over a direct write.

This also allows to keep track of the change through dynamic debug.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>