设置api发送的FM频道

        :params channel_num: channel_list的索引值 int

def set_channel(self, channel_num):
        """
        设置api发送的FM频道

        :params channel_num: channel_list的索引值 int
        """
        douban.set_channel(channel_num)
        self.empty()
        if channel_num != 2:
            self._get_first_song()