ALSA: ac97: set variables dev_attr_vendor_id to static

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.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>