Null UnitId

[Victor Scaletchi]

readSessionStartedPacket in garmin.py doesn't return the unit_id value. As a result, the import on both TrainingPeaks and MotionBase is failing, the UnitId tag in the .tcx file is populated with "None" (a numeric value is expected here).
The fix (see bellow) was to add the return line.

    def readSessionStartedPacket(self):
        """Read from the USB bus until session started packet is received."""
        session_started = False
        unit_id_data = None
        while not session_started:
            packet = self.readUSBPacket(16)
            if len(packet) != 16:
                continue
            # We have something that could be the right packet.
            packet_id, unit_id_data = self.unpack(packet)
            if packet_id in [self.Pid_Session_Started,
                             self.Pid_Session_Started2]:
                session_started = True
        [self.unit_id] = struct.unpack("<L", unit_id_data)
        return self.unit_id