PCI/physdev: address Misra 2.1 rule violation

Cases 0..3 are handled, and a 2-bit mask is applied to the switch()
expression. Therefore the default: case is reported unreachable by Eclair.
Insert BUILD_ERROR() to annotate this for Eclair.

No functional change.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

xen/drivers/pci/physdev.c

@@ -114,7 +114,7 @@ ret_t pci_physdev_op(int cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
             break;
 
         default:
-            ret = -EINVAL;
+            BUILD_ERROR("PCI_DEVICE_RESET_* inconsistency");
             break;
         }
         write_unlock(&pdev->domain->pci_lock);