power: supply: max1721x: Use strscpy() is more robust and safer

The implementation of strscpy() is more robust and safer.

That's now the recommended way to copy NUL terminated strings.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Minghao Chi <chi.minghao@zte.com.cn>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>