Rename DWCONV CHW microkernels to DWCONV2D CHW

Explicitly mention that microkernels assume 2D layout

PiperOrigin-RevId: 338788486
diff --git a/src/xnnpack/operator.h b/src/xnnpack/operator.h
index 5a5f42f..edc24ce 100644
--- a/src/xnnpack/operator.h
+++ b/src/xnnpack/operator.h
@@ -121,7 +121,7 @@
 // Direct 2D Depthwise Convolution
 struct xnn_ukernel_dwconv2d {
   union {
-    xnn_dwconv_chw_ukernel_function chw_function;
+    xnn_dwconv2d_chw_ukernel_function chw_function;
   };
   uint8_t input_width_tile;
   uint8_t output_width_tile;