ALSA: hda - Remove unnecessary caches for power states in patch_sigmatel.c

The power-state changes in patch_sigmatel.c are accessed via *_cached()
but they shouldn't be really cached.  Fixed to the normal write.

Also, stac92hd71xx_suspend and resume are no longer necessary as the
power-state changes are handled properly in the common routine.
Removed these hacks now.

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