alsa生成/dev/snd/下设备节点函数调用流程
snd_soc_register_card()->
 snd_soc_instantiate_card()->
  snd_card_register()->
   snd_device_register_all()->
    dev->ops->dev_register()->(函数指针)
     snd_pcm_dev_register->(赋值给函数指针 dev_register)
    snd_register_device_for_dev->(最终应用层通过snd_pcm_f_ops函数集去操作audio硬件）