passthrough: drop break statement following c/s cd7dedad82

The locking responsibilities have changed and a premature break in
this section now causes the following assertion:

Assertion '!preempt_count()' failed at preempt.c:36

Reported-by: Sander Eikelenboom <linux@eikelenboom.it>
Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Igor Druzhinin <igor.druzhinin@citrix.com>
Reviewed-by: Paul Durrant <pdurrant@amazon.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/drivers/passthrough/pci.c

@@ -1705,7 +1705,6 @@ int iommu_do_pci_domctl(
                        seg, bus, PCI_SLOT(devfn), PCI_FUNC(devfn));
                 ret = -EINVAL;
             }
-            break;
         }
         else if ( !ret )
             ret = assign_device(d, seg, bus, devfn, flags);