ALSA: compress: Fix kernel-doc warnings

Some fields in struct snd_compr have no corresponding comments, and
the kernel-doc complains like:

  ./include/sound/compress_driver.h:162: warning: No description found for parameter 'id[64]'
  ./include/sound/compress_driver.h:162: warning: No description found for parameter 'proc_root'
  ./include/sound/compress_driver.h:162: warning: No description found for parameter 'proc_info_entry'

Actually all these are internal elements, just put "private:" comment
so that they will be ignored.

Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>