ALSA: ac97: fix check of pm_runtime_get_sync failure

pm_runtime_get_sync returns negative on failure.

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