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>

drivers/regulator/bd9571mwv-regulator.c

@@ -217,7 +217,7 @@ static ssize_t backup_mode_store(struct device *dev,
 	return count;
 }
 
-DEVICE_ATTR_RW(backup_mode);
+static DEVICE_ATTR_RW(backup_mode);
 
 static int bd9571mwv_suspend(struct device *dev)
 {