[PATCH] libata: improve handling of diagostic fail (and hardware that misreports it)

Our ATA probe code checks that a device is not reporting a diagnostic
failure during start up. Unfortunately at least one device seems to like
doing this - the Gigabyte iRAM.

This is only done for the master right now (which is fine for the iRAM
as it is SATA), as with PATA some combinations of ATAPI device seem to
fool the check into seeing a drive that isn't there if it is applied to
the slave.

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>