ahci: Add ifdef wrapper to ahci_gtf_filter_workaround

Commit f80ae7e4
ahci: filter FPDMA non-zero offset enable for Aspire 3810T
breaks the current git build for configurations that don't define
CONFIG_ATA_ACPI.
This adds an ifdef wrapper to ahci_gtf_filter_workaround.

Signed-off-by: Markus Trippelsdorf <markus@trippelsdorf.de>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>