snd-hda-intel driver used schedule_work() fot the delayed DMA pointer
updates, but this has several potential problems:
- it may block other eventsd works longer
- it may deadlock when probing fails and flush_scheduled_work() is
  called during probe callback (as probe callback itself could be
  invoked from eventd)

This patch adds an own workq for each driver instance to solve these
problems.
Signed-off-by: Takashi Iwai <tiwai@suse.de>