Fixes the following sparse warning:

drivers/regulator/bd9571mwv-regulator.c:220:1: warning:
 symbol 'dev_attr_backup_mode' was not declared. Should it be static?
Signed-off-by: NWei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: NMark Brown <broonie@kernel.org>