klippy: Convert printer_state("disconnect") to an event handler

Convert all users of the printer_state("disconnect") handler to
register a "klippy:disconnect" event handler instead.

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

klippy/mcu.py

@@ -423,6 +423,8 @@ class MCU:
         if self._name.startswith('mcu '):
             self._name = self._name[4:]
         self._printer.register_event_handler("klippy:shutdown", self._shutdown)
+        self._printer.register_event_handler("klippy:disconnect",
+                                             self._disconnect)
         # Serial port
         self._serialport = config.get('serial', '/dev/ttyS0')
         baud = 0
@@ -768,8 +770,6 @@ class MCU:
     def printer_state(self, state):
         if state == 'connect':
             self._connect()
-        elif state == 'disconnect':
-            self._disconnect()
     def __del__(self):
         self._disconnect()