[SCSI] aacraid: Better handling of in-flight events on thread stop

When an error occured that would shut down the driver, some in-flight
events were getting caught up, deadlocking a CPU or two.

Signed-off-by: Ben Collins <bcollins@ubuntu.com>
Acked-by: Achim Leubner <Achim_Leubner@pmc-sierra.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>