extruder: Fix SET_PRESSURE_ADVANCE so that it works with multiple extruders

Use the new gcode.register_mux_command() so that SET_PRESSURE_ADVANCE
works correctly with multiple extruders.

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

klippy/toolhead.py

@@ -372,6 +372,8 @@ class ToolHead:
         self.extruder = extruder
         self.move_queue.set_extruder(extruder)
         self.commanded_pos[3] = extrude_pos
+    def get_extruder(self):
+        return self.extruder
     # Misc commands
     def stats(self, eventtime):
         for m in self.all_mcus: