mwifiex: prevent an array overflow

The "rate_index" is only used as an index into the phist_data->rx_rate[]
array in the mwifiex_hist_data_set() function.  That array has
MWIFIEX_MAX_AC_RX_RATES (74) elements and it's used to generate some
debugfs information.  The "rate_index" variable comes from the network
skb->data[] and it is a u8 so it's in the 0-255 range.  We need to cap
it to prevent an array overflow.

Fixes: cbf6e055 ("mwifiex: add rx histogram statistics support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/marvell/mwifiex/cfp.c

@@ -531,5 +531,8 @@ u8 mwifiex_adjust_data_rate(struct mwifiex_private *priv,
 		rate_index = (rx_rate > MWIFIEX_RATE_INDEX_OFDM0) ?
 			      rx_rate - 1 : rx_rate;
 
+	if (rate_index >= MWIFIEX_MAX_AC_RX_RATES)
+		rate_index = MWIFIEX_MAX_AC_RX_RATES - 1;
+
 	return rate_index;
 }