Convert the ac97_bus from legacy suspend/resume callbacks to dev_pm_ops.

Since there isn't anything special to do at the bus level the bus driver
does not have to implement any callbacks. The device driver core will
automatically pick up and execute the device's PM ops.

As there is only a single AC'97 driver implementing suspend and resume,
update both the core and driver at the same time to avoid unnecessary code
churn.

While we are at it also drop the ifdefs around the suspend/resume functions
to increase compile test coverage.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>