Fixed bug in acomms_driver init() when connection to modem isn't made before ros master is shutdown

Bugfixes

• fixes bug that happened when ros master is shutdown before the acomms_driver node is able to get a $CCCFQ,SRC reply $CACFG,SRC,X from the modem. Before this query was in a while True loop. Now it's in while not rospy.is_shutdown() and connected_to_modem = False to start and is only connected_to_modem = True when we got a reply and we break from the loop. After breaking out of this loop, we check if connected_to_modem. If we are, we proceed, Else, rospy.is_shutdown() is True so we print an error and return