ALSA: pcm: Use CLASS() for fdget()/fdput()

Now we have a nice definition of CLASS(fd) that can be applied as a
clean up for the fdget/fdput pairs in snd_pcm_link().

No functional changes, only code refactoring.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/20240223084241.3361-2-tiwai@suse.de