ath6kl: Check for valid rate table index

There are 28 items defined in rate table array 'wmi_rate_tbl'.
The rate table index (reply->rate_index) in ath6kl_wmi_bitrate_reply_rx()
func is not checked for the valid max limit index before accessing
rate table array. There may be some incidents to get memory crashes
without safe max check. Fix this.

Found this on code review.

Signed-off-by: Raja Mani <rmani@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>