wlan_platform: Add CONFIG_CNSS_OUT_OF_TREE flag

Use CONFIG_CNSS_OUT_OF_TREE to control if CNSS family drivers are
built from in-kernel-tree source or out-of-kernel-tree source since
the exposed header files will be in different paths. This is also
for backward compatibility.

Change-Id: I24111fdb76607bf911c29e415d42d25609f9b26c

cnss_prealloc/cnss_prealloc.c

@@ -7,7 +7,11 @@
 #include <linux/mm.h>
 #include <linux/err.h>
 #include <linux/of.h>
+#ifdef CONFIG_CNSS_OUT_OF_TREE
+#include "cnss_prealloc.h"
+#else
 #include <net/cnss_prealloc.h>
+#endif
 
 MODULE_LICENSE("GPL v2");
 MODULE_DESCRIPTION("CNSS prealloc driver");