regulator: bd9571mwv: Make symbol 'dev_attr_backup_mode' static

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: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Mark Brown <broonie@kernel.org>