ata-acpi: don't call _GTF for disabled drive

I got below log after a S3 resume in a ASUS A6VC laptop. The system has
only one IDE drive. It appears there is no reason calling _GTF for
disabled drive.

ACPI Error (dsopcode-0483): Attempt to CreateField of length zero [20070126]
ACPI Error (psparse-0537): Method parse/execution failed [\_SB_.PCI0.IDE0.RATA] (Node df822a7c), AE_AML_OPERAND_VALUE
ACPI Error (psparse-0537): Method parse/execution failed [\_SB_.PCI0.IDE0.CHN1.DRV0._GTF] (Node df822bd0), AE_AML_OPERAND_VALUE
ata2.00: _GTF evaluation failed (AE 0x3006)
ACPI Error (dsopcode-0483): Attempt to CreateField of length zero [20070126]
ACPI Error (psparse-0537): Method parse/execution failed [\_SB_.PCI0.IDE0.RATA] (Node df822a7c), AE_AML_OPERAND_VALUE
ACPI Error (psparse-0537): Method parse/execution failed [\_SB_.PCI0.IDE0.CHN1.DRV1._GTF] (Node df822b94), AE_AML_OPERAND_VALUE
ata2.01: _GTF evaluation failed (AE 0x3006)

Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>