Fix acceleration ramping crashing into endstops hard

If acceleration ramping is enabled Teacup tries to brake slowly when hitting an endstop. This seems to work with slow feedrates but will cause catastrophic failure when running fast and the endstop beeing a hard hit at the end of an axis (like the mechanical endstops on a mendel90, this nearly destroyed a plastic part here.)

Just removed the ramping on hitting an endstop and replaced it with an immediate stop.