ACPI / battery: Use design-cap for capacity calculations if full-cap is not available

The ThunderSoft TS178 tablet's _BIX implementation reports design_capacity
but not full_charge_capacity.

Before this commit this would cause us to return -ENODEV for the capacity
attribute, which userspace does not like. Specifically upower does this:

        if (sysfs_file_exists (native_path, "capacity")) {
                percentage = sysfs_get_double (native_path, "capacity");

Where the sysfs_get_double() helper returns 0 when we return -ENODEV,
so the battery always reads 0% if we return -ENODEV.

This commit fixes this by using the design-capacity instead of the
full-charge-capacity when the full-charge-capacity is not available.

Fixes: b41901a2cf ("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>

drivers/acpi/battery.c

@@ -217,7 +217,7 @@ static int acpi_battery_get_property(struct power_supply *psy,
 				     enum power_supply_property psp,
 				     union power_supply_propval *val)
 {
-	int ret = 0;
+	int full_capacity = ACPI_BATTERY_VALUE_UNKNOWN, ret = 0;
 	struct acpi_battery *battery = to_acpi_battery(psy);
 
 	if (acpi_battery_present(battery)) {
@@ -286,12 +286,17 @@ static int acpi_battery_get_property(struct power_supply *psy,
 			val->intval = battery->capacity_now * 1000;
 		break;
 	case POWER_SUPPLY_PROP_CAPACITY:
+		if (ACPI_BATTERY_CAPACITY_VALID(battery->full_charge_capacity))
+			full_capacity = battery->full_charge_capacity;
+		else if (ACPI_BATTERY_CAPACITY_VALID(battery->design_capacity))
+			full_capacity = battery->design_capacity;
+
 		if (battery->capacity_now == ACPI_BATTERY_VALUE_UNKNOWN ||
-		    !ACPI_BATTERY_CAPACITY_VALID(battery->full_charge_capacity))
+		    full_capacity == ACPI_BATTERY_VALUE_UNKNOWN)
 			ret = -ENODEV;
 		else
 			val->intval = battery->capacity_now * 100/
-					battery->full_charge_capacity;
+					full_capacity;
 		break;
 	case POWER_SUPPLY_PROP_CAPACITY_LEVEL:
 		if (battery->state & ACPI_BATTERY_STATE_CRITICAL)