[SCSI] megaraid_sas: Fix the fix for fw hang caused by megaraid sas application

Add a lock to the skinny firmware initialisation sequence to prevent
the two stage write being non atomic if multiple instances use it.

Add a flag to the driver shutdown sequence to prevent aen ioctls being
called after shutdown begins.

Signed-off-by Bo Yang<bo.yang@lsi.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>