power: supply: use sysfs_emit() instead of scnprintf() for sysfs show()

As documented in Documentation/filesystems/sysfs.rst the sysfs
show() function should use sysfs_emit() or sysfs_emit_at() to
format the userspace return value. This replaces all instances
of scnprintf() with sysfs_emit() in the power-supply subsystem.

Signed-off-by: ye xingchen <ye.xingchen@zte.com.cn>
[reword commit message]
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>