Commit b41901a2 ("ACPI / battery: Do not export energy_full[_design]
on devices without full_charge_capacity") added support for some (broken)
devices which always report 0 for both design- and full_charge-capacity.

This assumes that if the capacity is not being reported it is 0. The
ThunderSoft TS178 tablet's _BIX implementation falsifies this assumption.
It reports ACPI_BATTERY_VALUE_UNKNOWN (-1) as full_charge_capacity, which
we treat as a valid value which causes several problems.

This commit fixes this by adding a new ACPI_BATTERY_CAPACITY_VALID() helper
which checks that the value is not 0 and not -1; and using this whenever we
need to test if either design_capacity or full_charge_capacity is valid.

Fixes: b41901a2 ("ACPI / battery: Do not export energy_full[_design] on devices without full_charge_capacity")
Cc: 4.19+ <stable@vger.kernel.org> # 4.19+
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>