scsi: aacraid: Block concurrent hotplug event handling

Currently driver will attempt to process hotplug events concurrently based
on the FW interrupt.

Protect safw update function with a scan mutex.

Signed-off-by: Raghava Aditya Renukunta <RaghavaAditya.Renukunta@microsemi.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>