libata-acpi: fix _GTF command protocol for ATAPI devices

_GTF command is never ATA_PROT_ATAPI_NODATA whether the device is
ATAPI or not.  It's always ATA_PROT_NODATA.

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