libata: consider errors not associated with commands for speed down

libata EH used to ignore errors not associated with commands when
determining whether speed down is necessary or not.  This leads to the
following problems.

* Errors not associated with commands can occur indefinitely without
  libata EH taking corrective actions.

* Upstream link errors don't trigger speed down when PMP is attached
  to it and commands issued to downstream device trigger errors on the
  upstream link.

This patch makes ata_eh_link_autopsy() consider errors not associated
with command for speed down.

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