xen/arm: use is_device_dma_coherent

Use is_device_dma_coherent to check whether we need to issue cache
maintenance operations rather than checking on the existence of a
particular dma_ops function for the device.

This is correct because coherent devices don't need cache maintenance
operations - arm_coherent_dma_ops does not set the hooks that we
were previously checking for existance.
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

arch/arm/xen/mm32.c

@@ -48,7 +48,7 @@ void __xen_dma_unmap_page(struct device *hwdev, dma_addr_t handle,
 		struct dma_attrs *attrs)
 
 {
-	if (!__generic_dma_ops(hwdev)->unmap_page)
+	if (is_device_dma_coherent(hwdev))
 		return;
 	if (dma_get_attr(DMA_ATTR_SKIP_CPU_SYNC, attrs))
 		return;
@@ -59,7 +59,7 @@ void __xen_dma_unmap_page(struct device *hwdev, dma_addr_t handle,
 void __xen_dma_sync_single_for_cpu(struct device *hwdev,
 		dma_addr_t handle, size_t size, enum dma_data_direction dir)
 {
-	if (!__generic_dma_ops(hwdev)->sync_single_for_cpu)
+	if (is_device_dma_coherent(hwdev))
 		return;
 	__xen_dma_page_dev_to_cpu(hwdev, handle, size, dir);
 }
@@ -67,7 +67,7 @@ void __xen_dma_sync_single_for_cpu(struct device *hwdev,
 void __xen_dma_sync_single_for_device(struct device *hwdev,
 		dma_addr_t handle, size_t size, enum dma_data_direction dir)
 {
-	if (!__generic_dma_ops(hwdev)->sync_single_for_device)
+	if (is_device_dma_coherent(hwdev))
 		return;
 	__xen_dma_page_cpu_to_dev(hwdev, handle, size, dir);
 }