wcn36xx: Fix warning due to bad rate_idx

The rate_idx is the index of the bitrate in the supported rate table.
However the 5Ghz band has a smaller legacy bitrate table than 2.4Ghz
since it does not have the DSSS bitrates (1, 2, 5.5, 11).

So in 5Ghz band the index should adjusted accrodingly (-4).

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
[bod: Made sure fix is only applied if the rate_idx > n_bitrates]
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200829033846.2167619-6-bryan.odonoghue@linaro.org