ALSA: hda - Handle get/set power verb symmetrically via regmap

HD-audio has quite a few asymmetrical ways of accessing verbs, and one
of typical ones is GET/SET_POWER_STATE verbs.  While it takes only the
power state for setting, it returns a combination of states for
getting.  For making the state handling simpler, this patch adds a
code to translate the value returned from GET_POWER_STATE to return
only the actual state or -1 for error.  In that way, the driver can
simplify the power state management.

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