cfg80211: Validate legacy rateset.

Legacy rates are not validated while configuring
tx rateset using iw. So below cmd is accepted by nl80211.
sudo iw wlan2 set bitrates legacy-2.4 1 2 3

Validate legacy rates and return
error if any rate in the rateset is not valid.

Signed-off-by: Bala Shanmugam <bkamatch@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/wireless/nl80211.c

@@ -5577,6 +5577,9 @@ static int nl80211_set_tx_bitrate_mask(struct sk_buff *skb,
 				sband,
 				nla_data(tb[NL80211_TXRATE_LEGACY]),
 				nla_len(tb[NL80211_TXRATE_LEGACY]));
+			if ((mask.control[band].legacy == 0) &&
+			    nla_len(tb[NL80211_TXRATE_LEGACY]))
+				return -EINVAL;
 		}
 		if (tb[NL80211_TXRATE_MCS]) {
 			if (!ht_rateset_to_mask(