mfd: Use sysfs_emit() to instead of scnprintf()

Follow the advice of the Documentation/filesystems/sysfs.rst and show()
should only use sysfs_emit() or sysfs_emit_at() when formatting the
value to be returned to user space.

Signed-off-by: ye xingchen <ye.xingchen@zte.com.cn>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/202212021528368292334@zte.com.cn