Make mikrotik method setting optional as intended (#18454)

Pull request #17852 accidently made the `method` setting required.

homeassistant/components/device_tracker/mikrotik.py

@@ -46,7 +46,7 @@ class MikrotikScanner(DeviceScanner):
         self.port = config[CONF_PORT]
         self.username = config[CONF_USERNAME]
         self.password = config[CONF_PASSWORD]
-        self.method = config[CONF_METHOD]
+        self.method = config.get(CONF_METHOD)
 
         self.connected = False
         self.success_init = False