libata: implement ATA_QCFLAG_RETRY

Currently whether a command should be retried after failure is
determined inside ata_eh_finish().  Add ATA_QCFLAG_RETRY and move the
logic into ata_eh_autopsy().  This makes things clearer and helps
extending retry determination logic.

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