libata: IORDY handling

I believe this version meets all Sergei's objections

Correct the logic for when we issue a set features for transfer mode

- If the device has IORDY and the controller has IORDY - set the mode
- If the device has IORDY and the controller does not - turn IORDY off
- If neither has IORDY do nothing

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>