[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: Mark Haverkamp <markh@osdl.org>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>