Fix for port argument

mqtt_sniffer.py

 if __name__ == "__main__":
     parser = argparse.ArgumentParser(description='This is a mqtt sniffer.')
     parser.add_argument('-f', dest='hostname', default='localhost', help='Hostname of the mqtt server')
-    parser.add_argument('-p', dest='port', default=1883, help='Port of the mqtt server')
+    parser.add_argument('-p', dest='port', default=1883, type=int, help='Port of the mqtt server')
     parser.add_argument('-n', dest='no_credentials', action='store_true', help='Avoid asking for credentials')
     parser.add_argument('-u', dest='username', default=None, help='Set username for mqtt server')
     parser.add_argument('-t', dest='topicfilter', default="", help='Set topic filter')