wifi: cfg80211: make cfg80211_auth_request::key_idx signed

We might assign -1 to it in some cases when key is NULL,
which means the key_idx isn't used but can lead to a
warning from static checkers such as smatch.

Make the struct member signed simply to avoid that, we
only need a range of -1..3 anyway.
Signed-off-by: NJohannes Berg <johannes.berg@intel.com>

include/net/cfg80211.h

@@ -2773,7 +2773,8 @@ struct cfg80211_auth_request {
 	size_t ie_len;
 	enum nl80211_auth_type auth_type;
 	const u8 *key;
-	u8 key_len, key_idx;
+	u8 key_len;
+	s8 key_idx;
 	const u8 *auth_data;
 	size_t auth_data_len;
 	s8 link_id;