regulator: bd9571mwv: Convert device attribute to sysfs_emit()

Convert the "backup_mode" device attribute from sprintf() to
sysfs_emit(), as the latter is aware of the PAGE_SIZE buffer.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20210312130242.3390038-4-geert+renesas@glider.be


Signed-off-by: Mark Brown <broonie@kernel.org>