scsi: aacraid: change event_wait to a completion

The event_wait semaphore has completion semantics, so we can
change it over to the completion interface for clarity without
changing the behavior.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Dave Carroll <david.carroll@microsemi.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>