update rexm switch search routine

The procedure to accurately position the rda has been updated by winlight.
Basically instead of :

• moving at at 3.2mm/sec in one direction until it hit the switch
• backing off at 1.1mm/sec
• move back to the switch at 1.1mm/sec

Now, it will go all the way at 3.2mm, decelerate at 10,24 mm/sec² once the switch is hit, this will give the 500microns to fully engage the switch.