libata: adjust speed down rules

Speed down rules were too conservative.  Adjust them a bit.

* More than 10 timeouts can't happen in 5 minutes as command timeout
  is 30secs.  Lower the limit for rule #1 to 6.

* 10 timeouts is too high for rule #3 too.  Lower it to 6.

* SATAPI can benefit from falling back to PIO too.  Allow SATAPI
  devices to fall back to PIO.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>