net: dsa: mv88e6xxx: remove bridge work

Now that the bridge code defers the switchdev port state setting, there
is no need to defer the port STP state change within the mv88e6xxx code.
Thus get rid of the driver's bridge work code.

This also fixes a race condition where the DSA layer assumes that the
bridge code already set the unbridged port's STP state to Disabled
before restoring the Forwarding state.

As a consequence, this also fixes the FDB flush for the unbridged port
which now correctly occurs during the Forwarding to Disabled transition.

Fixes: 0bc05d58 ("switchdev: allow caller to explicitly request attr_set as deferred")
Reported-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>