passthrough: fix xsm-related oversight

Presumably a copy-and-paste mistake, which I also didn't notice while
unifying x86's and ia64's respective domctl implementations.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/drivers/passthrough/iommu.c
+++ b/xen/drivers/passthrough/iommu.c
@@ -625,7 +625,7 @@ int iommu_do_domctl(
             break;
         }
 
-        ret = xsm_assign_device(d, domctl->u.assign_device.machine_sbdf);
+        ret = xsm_deassign_device(d, domctl->u.assign_device.machine_sbdf);
         if ( ret )
             goto deassign_device_out;