net: dsa: rollback bridging on error

When an error is returned during the bridging of a port in a
NETDEV_CHANGEUPPER event, net/core/dev.c rolls back the operation.

Be consistent and unassign dp->bridge_dev when this happens.

In the meantime, add comments to document this behavior.

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