ALSA: hda: Flush pending unsolicited events before suspend

ALSA: hda: Flush pending unsolicited events before suspend

The HD-audio controller driver processes the unsolicited events via
its work asynchronously, and this might be pending when the system
goes to suspend.  When a lengthy event handling like ELD byte reads is
running, this might trigger unexpected accesses among suspend/resume
procedure, typically seen with Nvidia driver that still requires the
handling via unsolicited event verbs for ELD updates.

This patch adds the flush of unsol_work to assure that pending events
are processed before going into suspend.

Buglink: https://bugzilla.suse.com/show_bug.cgi?id=1182377
Reported-and-tested-by: Abhishek Sahu <abhsahu@nvidia.com>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20210310112809.9215-2-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>