minstrel_ht: make *idx unsigned in minstrel_downgrade_rate

net/mac80211/rc80211_minstrel_ht.c:440:46: warning: incorrect type in argument 2 (different signedness)
net/mac80211/rc80211_minstrel_ht.c:440:46:    expected int *idx
net/mac80211/rc80211_minstrel_ht.c:440:46:    got unsigned int *<noident>
net/mac80211/rc80211_minstrel_ht.c:446:46: warning: incorrect type in argument 2 (different signedness)
net/mac80211/rc80211_minstrel_ht.c:446:46:    expected int *idx
net/mac80211/rc80211_minstrel_ht.c:446:46:    got unsigned int *<noident>

Signed-off-by: John W. Linville <linville@tuxdriver.com>
Acked-by: Felix Fietkau <nbd@openwrt.org>

net/mac80211/rc80211_minstrel_ht.c

@@ -328,7 +328,8 @@ minstrel_next_sample_idx(struct minstrel_ht_sta *mi)
 }
 
 static void
-minstrel_downgrade_rate(struct minstrel_ht_sta *mi, int *idx, bool primary)
+minstrel_downgrade_rate(struct minstrel_ht_sta *mi, unsigned int *idx,
+			bool primary)
 {
 	int group, orig_group;