Added RC setpoint control in H-H mode
[rotorcraft] fix hover speed RC setpoint

- fix rc_x and rc_y values
  - still needs to be scaled to max speed using circle instead of box bounding,
    but while making sure a division by zero can't occur!
- only copy pos_ref to pos_sp in hover mode
- move GUIDANCE_H_REF_MAX_SPEED|ACCEL to ref header file so it can be used
- refactor rc speed setpoint reading into separate function

closes #543