mlw_codec: Fixed alignment warning

- Restructured pointer API to prevent alignment warnings
- Changed weight tensor data type to np.int16

Change-Id: I310c1ca733bf98724c84e8b2194becb4be3e7eea
diff --git a/ethosu/mlw_codec/mlw_encode.h b/ethosu/mlw_codec/mlw_encode.h
index 743603b..68218f3 100644
--- a/ethosu/mlw_codec/mlw_encode.h
+++ b/ethosu/mlw_codec/mlw_encode.h
@@ -47,7 +47,7 @@
     int kernel_width,
     int ifm_depth,
     int* brick_strides,
-    void* inbuf,
+    int16_t* inbuf,
     int ofm_block_depth,
     int is_depthwise,
     int is_partkernel,