scsi: aacraid: use scsi_host_busy_iter() to wait for outstanding commands

Instead of traversing the list of possible commands by hands we should be
using scsi_host_busy_iter() to figure out if there are outstanding
commands.

Link: https://lore.kernel.org/r/20200228075318.91255-12-hare@suse.de
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Balsundar P &lt; <Balsundar.P@microchip.com>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>