ALSA: ac97: fix device initialization in the compat layer

ac97->dev is an object of 'struct device' type. It should be initialized
via device_initialize() or device_register().

Fixes: 74426fbf ("ALSA: ac97: add an ac97 bus")
Signed-off-by: Lihua Yao <ylhuajnu@163.com>
Tested-by: Robert Jarzmik <robert.jarzmik@free.fr>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>