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>