iommu/renesas: Fix unused-function warning

gcc warns because the only reference to ipmmu_find_group
is inside of an #ifdef:

drivers/iommu/ipmmu-vmsa.c:878:28: error: 'ipmmu_find_group' defined but not used [-Werror=unused-function]

Change the #ifdef to an equivalent IS_ENABLED().

Fixes: 6580c8a ("iommu/renesas: Convert to probe/release_device() call-backs")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Simon Horman <horms@verge.net.au>
Link: https://lore.kernel.org/r/20200508220224.688985-1-arnd@arndb.de
Signed-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/ipmmu-vmsa.c

@@ -903,11 +903,8 @@ static const struct iommu_ops ipmmu_ops = {
 	.probe_device = ipmmu_probe_device,
 	.release_device = ipmmu_release_device,
 	.probe_finalize = ipmmu_probe_finalize,
-#if defined(CONFIG_ARM) && !defined(CONFIG_IOMMU_DMA)
-	.device_group = generic_device_group,
-#else
-	.device_group = ipmmu_find_group,
-#endif
+	.device_group = IS_ENABLED(CONFIG_ARM) && !IS_ENABLED(CONFIG_IOMMU_DMA)
+			? generic_device_group : ipmmu_find_group,
 	.pgsize_bitmap = SZ_1G | SZ_2M | SZ_4K,
 	.of_xlate = ipmmu_of_xlate,
 };