libata-sff: fix spurious IRQ handling

Commit 27943620 introduced spurious
IRQ handling but it has a race condition where valid completion can be
lost while trying to clear spurious IRQ leading to occassional command
timeouts.

This patch improves SFF interrupt handler such that

1. Once BMDMA HSM is stopped, the condition is never considered
   spurious.  As there's no way to resume stopped BMDMA HSM, if device
   status doesn't agree with BMDMA status, the only way out is
   aborting the command (otherwise, it will just end up timing out).

2. ap->ops->sff_check_status() can be safely called to clear spurious
   device IRQ as it atomically returns completion status but BMDMA IRQ
   status can't be cleared in safe way if command is in flight.  After
   a spurious IRQ, call ap->ops->sff_irq_clear() only if the
   respective device is idle and retry completion if
   sff_check_status() indicates command completion.

Please note that ata_piix uses bmdma_status for sff_irq_check() and #2
won't weaken spurious IRQ handling even with in-flight command because
if bmdma_status indicates IRQ pending but device status is not on
spurious check, the next IRQ handler invocation will abort the command
due to #1.

This fixes bko#15537.

   https://bugzilla.kernel.org/show_bug.cgi?id=15537



Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Andrew Benton <b3nton@gmail.com>
Cc: Petr Uzel <petr.uzel@centrum.cz>
Cc: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>