libata: ata_timing_compute() fixup

XFER_SW_DMA_0 mode should be excluded from the extended cycle timing
computations.

Acked-by: Alan Cox <alan@linux.intel.com>
Noticed-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>