Use monotonic clock for last_sae_token_key_update

Just the usual, with a new function os_reltime_initialized()
thrown in that checks whether time has ever been retrieved
(time can't be completely zero).

Signed-hostap: Johannes Berg <johannes.berg@intel.com>

src/utils/os.h

@@ -108,6 +108,12 @@ static inline int os_reltime_expired(struct os_reltime *now,
 }
 
 
+static inline int os_reltime_initialized(struct os_reltime *t)
+{
+	return t->sec != 0 || t->usec != 0;
+}
+
+
 /**
  * os_mktime - Convert broken-down time into seconds since 1970-01-01
  * @year: Four digit year