ACPI: battery: negate current when discharging

ACPI: battery: negate current when discharging

The ACPI specification requires that battery rate is always positive,
but the kernel ABI for POWER_SUPPLY_PROP_CURRENT_NOW
(Documentation/ABI/testing/sysfs-class-power) specifies that it should
be negative when a battery is discharging. When reporting CURRENT_NOW,
massage the value to match the documented ABI.

This only changes the sign of `current_now` and not `power_now` because
documentation doesn't describe any particular meaning for `power_now` so
leaving `power_now` unchanged is less likely to confuse userspace
unnecessarily, whereas becoming consistent with the documented ABI is
worth potentially confusing clients that read `current_now`.

Signed-off-by: Peter Marheine <pmarheine@chromium.org>
Link: https://patch.msgid.link/20250508024146.1436129-1-pmarheine@chromium.org
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>