qcacld-2.0: Avoid integer underflow in dot11f_unpack functions

propagation from qcacld-3.0 to qcacld-2.0

In function dot11f_unpack functions length of buffer passed is
decremented as pointer advances in the buffer. Add a check for
integer underflow before decrementing the length.

qcacld-3.0 to CLD2.0 propagation of the following CRs:
2237141, 2231755, 2246426, 2150438

Change-Id: I4ed39d326855c2027ff0bc3cbe5c8981a2ae2aa1
CRs-Fixed: 2259649

drivers/net/wireless/qcacld-2.0/CORE/MAC/src/include/dot11f.h

@@ -36,7 +36,7 @@
   *
   *
   * This file was automatically generated by 'framesc'
-  * Thu Dec 28 13:33:15 2017 from the following file(s):
+  * Wed Jun 20 14:32:20 2018 from the following file(s):
   *
   * dot11f.frms
   *
@@ -55,6 +55,10 @@ typedef tANI_U32 tDOT11F_U64[2];
 #   pragma warning (disable: 4214) /* nonstandard extension used */
 #endif /* Microsoft C/C++             bit field types other than int */
 
+#if !defined unlikely
+#define unlikely(x)    (x)
+#endif
+
 /*
  * Frames Return Codes:
  *