ALSA: ac97: set variables dev_attr_vendor_id to static

sparse reports
sound/ac97/bus.c:465:1: sparse: sparse: symbol 'dev_attr_vendor_id' was not declared. Should it be static?

This variable is only used in its defining file, so it should be static.
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202307242300.Oy0Dp2QI-lkp@intel.com/Signed-off-by: Yu Liao <liaoyu15@huawei.com>
Link: https://lore.kernel.org/r/20230802022649.2514787-1-liaoyu15@huawei.comSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/ac97/bus.c

@@ -462,7 +462,7 @@ static ssize_t vendor_id_show(struct device *dev,
 
 	return sysfs_emit(buf, "%08x", codec->vendor_id);
 }
-DEVICE_ATTR_RO(vendor_id);
+static DEVICE_ATTR_RO(vendor_id);
 
 static struct attribute *ac97_dev_attrs[] = {
 	&dev_attr_vendor_id.attr,