qcacld-3.0: Change the underlying type of mac_handle_t

Currently mac_handle_t is defined as a void pointer. This is
convenient from an information hiding point of view since that means a
non-MAC component cannot dereference a MAC handle to access MAC
private data. However this is not convenient from a defect prevention
point of view since the C standard allows any other pointer type to be
freely and silently converted to and from a void pointer, and hence
the compiler is unable to detect when a MAC handle is used in a
context where a different pointer type is expected.

An example of multiple such defects was addressed by Change-Id
I01812b2390269805da4d1a5cb40a811d1e22ec56 (qcacld-3.0: Fix bad
pointers being passed from SME to WMA).

To help prevent these kinds of defects change the definition of
mac_handle_t to be a pointer to an opaque struct.

Change-Id: I72483bf0e693d6eca24355f31d3a1653b8f31302
CRs-Fixed: 2268814

core/mac/inc/sir_types.h

@@ -42,10 +42,12 @@
  * pointer through this handle.
  */
 /*
- * NOTE WELL: an upcoming change will replace the void * with an
- * opaque pointer just as is currently done with hdd_handle_t
+ * NOTE WELL: struct opaque_mac_handle is not defined anywhere. This
+ * reference is used to help ensure that a mac_handle_t is never used
+ * where a different handle type is expected
  */
-typedef void *mac_handle_t;
+struct opaque_mac_handle;
+typedef struct opaque_mac_handle *mac_handle_t;
 
 /* retain legacy name until all instances have been replaced */
 typedef mac_handle_t tHalHandle;