ns_list: avoid UINT_FAST8_MAX

Prevent compilation issues when someone has included <stdint.h> before
a header file that needs to include <ns_list.h>.

Some toolchains like ARM C 5 will not provide UINT_FAST8_MAX in C++
unless __STDC_LIMIT_MACROS is defined, and if this was not defined the
first time <stdint.h> was included, it's too late.

We can get the maximum value for our unsigned list offset by casting -1
to it, thanks to modulo arithmetic.

features/frameworks/nanostack-libservice/mbed-client-libservice/ns_list.h

@@ -148,7 +148,7 @@ union \
 { \
     ns_list_t slist; \
     NS_FUNNY_COMPARE_OK \
-    NS_STATIC_ASSERT(link_offset <= UINT_FAST8_MAX, "link offset too large") \
+    NS_STATIC_ASSERT(link_offset <= (ns_list_offset_t) -1, "link offset too large") \
     NS_FUNNY_COMPARE_RESTORE \
     char (*offset)[link_offset + 1]; \
     entry_type *type; \