rocker: push struct switchdev_trans down through rocker code

There will be needed to have switchdev_trans available down in the call
chain, so propagate it instead of trans phase enum. This enum will be
removed anyway. Also, use prepare/commit phase check helpers to get
information about current phase of transaction.
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>