Commit 3d3b87f9 renamed last_sg_flush_time to min_restart_time to
ensure that flush_step_generation() would fully flush out moves
generated from the force_move module. However, now that force_move
calls note_mcu_movequeue_activity() with is_step_gen=True, this is no
longer necessary.
Rework min_restart_time to last_step_gen_time.
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>