libata: fix CFA handling in ide_timing_compute()

Use standard cycle timing for CFA PIO5 and PIO6 modes.

Based on commit 74638c84 for IDE subsystem.

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>