libata: fix internal command failure handling

When an internal command fails, it should be failed directly without
invoking EH.  In the original implemetation, this was accomplished by
letting internal command bypass failure handling in ata_qc_complete().
However, later changes added post-successful-completion handling to
that code path and the success path is no longer adequate as internal
command failure path.  One of the visible problems is that internal
command failure due to timeout or other freeze conditions would
spuriously trigger WARN_ON_ONCE() in the success path.

This patch updates failure path such that internal command failure
handling is contained there.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: stable@kernel.org
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>