libata-acpi: adjust constness in ata_acpi_gtm/stm() parameters

* No internal function uses const ata_port.  Drop const from @ap.

* Make ata_acpi_stm() copy @stm before using it and change @stm to
  const.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>