Use charge_control_*_threshold instead of charge_*_threshold

This is now documented as the standard Linux kernel API for charging thresholds: https://github.com/torvalds/linux/blob/master/Documentation/ABI/testing/sysfs-class-power

This was added to the thinkpad_acpi driver in https://github.com/torvalds/linux/commit/e33929537b76486d2ed576a0d9ce3ebff51bf851

This will break the feature on Thinkpads with a kernel older than that, but will also work with other drivers that support charging thresholds.