net: dsa: mv88e6xxx: add port private structure

Add a per-port mv88e6xxx_priv_port structure to store per-port related
data, instead of adding several arrays of DSA_MAX_PORTS elements in the
mv88e6xxx_priv_state structure.

It currently only contains the port STP state.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>