ACPI / AC: Remove struct acpi_device pointer from struct acpi_ac

Now the pointer of struct acpi_device can be got by
ACPI_COMPANION(struct acpi_ac->pdev->dev). So the pointer
is not necessary and remove it.

Signed-off-by: Lan Tianyu <tianyu.lan@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>