libata: no need to speed down if already at PIO0

After reset, transfer mode is always PIO0 regardless of
dev->xfer_mask.  Check dev->pio_mode before trying to slow down after
configuration failure.  This prevents bogus speed down before device
is actually configured.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Acked-by: Alan Cox <alan@redhat.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>