minstrel: don't complain about feedback for unrequested rates

"It's not problematic if minstrel gets feedback for rates that it
doesn't have in its list, it should just ignore it. - Felix"
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>
Cc: Felix Fietkau <nbd@openwrt.org>

net/mac80211/rc80211_minstrel.c

@@ -67,7 +67,6 @@ rix_to_ndx(struct minstrel_sta_info *mi, int rix)
 	for (i = rix; i >= 0; i--)
 		if (mi->r[i].rix == rix)
 			break;
-	WARN_ON(i < 0);
 	return i;
 }