ALSA: hda - Fix misc workqueue issues

Some fixes regarding snd-hda-intel workqueue:
- Use create_singlethread_workqueue() instead of create_workqueue()
  as per-CPU work isn't required.
- Allocate workq name string properly
- Renamed the workq name to "hd-audio*" to be more obvious.

Signed-off-by: Takashi Iwai <tiwai@suse.de>