output_pin: Improve GCodeRequestQueue timing on duplicate requests

If there is a duplicate request it is not necessary to add a 100ms
delay to the next update.  Rework the callback signaling to better
report these duplicate updates.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/extras/servo.py

@@ -45,10 +45,10 @@ class PrinterServo:
     def get_status(self, eventtime):
         return {'value': self.last_value}
     def _set_pwm(self, print_time, value):
-        if value != self.last_value:
-            self.last_value = value
-            self.mcu_servo.set_pwm(print_time, value)
-        return (True, 0.)
+        if value == self.last_value:
+            return "discard", 0.
+        self.last_value = value
+        self.mcu_servo.set_pwm(print_time, value)
     def _get_pwm_from_angle(self, angle):
         angle = max(0., min(self.max_angle, angle))
         width = self.min_width + angle * self.angle_to_width