[ALSA] hda-intel - Add flush_scheduled_work() in snd_hda_codec_free()

Added flush_scheduled_work() in snd_hda_codec_free() to make sure that
the all work is gone.  Also, optimized the condition to schedule the
delayed work in snd_hda_power_down().

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>