Asynchronous notification on ICH9 didn't work because it didn't write
AN FIS into the RX area - it only updates SNotification. Also,
snooping SDB_FIS RX area is racy against further SDB FIS receptions.
Let sata_async_notification() determine using SNTF if it's available
and snoop RX area iff SNTF isn't available
Signed-off-by: Tejun Heo <htejun@gmail.com> Cc: Kristen Carlson Accardi <kristen.c.accardi@intel.com> Signed-off-by: Jeff Garzik <jeff@garzik.org>