[ALSA] ac97 - Suppress power-saving mode on non-supporting drivers

[ALSA] ac97 - Suppress power-saving mode on non-supporting drivers

Don't enable power-saving mode on drivers that don't support
it.  The supporting drivers set AC97_SCAP_POWER_SAVE to scaps
at creation of ac97 instance.
Currently enable on the following drivers: intel8x0, intel8x0m,
atiixp, atiixp-modem, via82xx and via82xx-modem.
Also, a bit clean up of power-saving stuff:
- Don't create an own workq
- Remove superfluous ifdefs

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