libata: assume ATA_DEV_ATA on diagnostic failure

Certain device which reports diagnostic failure also reports invalid
device signature.  Assume ATA_DEV_ATA on diagnostic failure if reset
indicates device presence.

This is fix for bugzilla bug 8784.

  http://bugzilla.kernel.org/show_bug.cgi?id=8784

Signed-off-by: Tejun Heo <htejun@gmail.com>
Acked-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Edward Amsden <amsden_linux@earthlink.net>
Signed-off-by: Jeff Garzik <jeff@garzik.org>