scsi: qla2xxx: fix a potential NULL pointer dereference

alloc_workqueue is not checked for errors and as a result a potential
NULL dereference could occur.

Link: https://lore.kernel.org/r/1568824618-4366-1-git-send-email-allen.pais@oracle.com
Signed-off-by: Allen Pais <allen.pais@oracle.com>
Reviewed-by: Martin Wilck <mwilck@suse.com>
Acked-by: Himanshu Madhani <hmadhani@marvell.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
