[SCSI] aacraid: Fix extra unregister_chrdev

Received from Mark Salyzyn

If there are no aacraid controllers, we do not create the raid
controller chrdev, thus when the driver is unloaded it performs a
superfluous deregistration.
Signed-off-by: NMark Haverkamp <markh@osdl.org>
Signed-off-by: NJames Bottomley <James.Bottomley@SteelEye.com>

drivers/scsi/aacraid/linit.c

@@ -1023,7 +1023,8 @@ static int __init aac_init(void)
 
 static void __exit aac_exit(void)
 {
-	unregister_chrdev(aac_cfg_major, "aac");
+	if (aac_cfg_major > -1)
+		unregister_chrdev(aac_cfg_major, "aac");
 	pci_unregister_driver(&aac_pci_driver);
 }