iommu/irq_remapping: Refine function irq_remapping_prepare() for maintenance

Assign intel_irq_remap_ops to remap_ops only if
intel_irq_remap_ops.prepare() succeeds.
Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Tested-by: Joerg Roedel <joro@8bytes.org>
Cc: Tony Luck <tony.luck@intel.com>
Cc: iommu@lists.linux-foundation.org
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Link: http://lkml.kernel.org/r/1420615903-28253-15-git-send-email-jiang.liu@linux.intel.comSigned-off-by: Thomas Gleixner <tglx@linutronix.de>

drivers/iommu/irq_remapping.c

@@ -204,15 +204,15 @@ int __init irq_remapping_prepare(void)
 	if (disable_irq_remap)
 		return -ENOSYS;
 
+	if (intel_irq_remap_ops.prepare() == 0)
 		remap_ops = &intel_irq_remap_ops;
-
-#ifdef CONFIG_AMD_IOMMU
-	if (amd_iommu_irq_ops.prepare() == 0) {
+	else if (IS_ENABLED(CONFIG_AMD_IOMMU) &&
+		 amd_iommu_irq_ops.prepare() == 0)
 		remap_ops = &amd_iommu_irq_ops;
+	else
+		return -ENOSYS;
+
 	return 0;
-	}
-#endif
-	return remap_ops->prepare();
 }
 
 int __init irq_remapping_enable(void)