[SCSI] aacraid: fix potential panic in thread stop

Got a panic in the threading code on an older kernel when the Adapter
failed to load properly and driver shut down apparently before any
threading had started, can not dupe. Expect that this may be relevant in
the latest kernel, but not sure. This patch does no harm, and should
alleviate the possibility of this panic.

Signed-off-by: Mark Salyzyn <aacraid@adaptec.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>