[ALSA] sound/pci/hda: use create_singlethread_workqueue()

process_unsol_events() seems to assume a singlethread one (IOW, racey).
So, this patch uses create_singlethread_workqueue() instead of
create_workqueue().

Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>