BUGFIX to USB-HOMING

* wou_stepgen.c: merge MT_RISC_CMD into MT_PROBED_POS
* usb_homing.c:
  + do not check home_sw_active at *_WAIT state,
    let RISC-PROBE do the job
  + modify the RISC-PROBE polarity for home_start_move()
    because, in arte, all of our home switches are active-low
  * add debug message to detect home_state change