wcn36xx: fix RX BD rate mapping for 5GHz legacy rates

The linear mapping between the BD rate field and the driver's 5GHz
legacy rates table (wcn_5ghz_rates) does not only apply for the latter
four rates -- it applies to all eight rates.

Fixes: 6ea131ac ("wcn36xx: Fix warning due to bad rate_idx")
Signed-off-by: Benjamin Li <benl@squareup.com>
Tested-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20211104010548.1107405-3-benl@squareup.com