net: dsa: mv88e6xxx: use bridge state values

Reuse the BR_STATE_* values to abstract a port STP state value.

This provides shorter names and better control over the DSA switch
operation call.

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