net80211: fix a typo in Rx MCS set for unequal modulation case

[misha]

Summary

RX MCS set defines which MCSs are supported for RX, bits 0-31 are for equal modulation of the streams, bits 33-76 are for unequal case. Current code checks txstreams variable instead of rxstreams to set bits from 53 to 76 for 4 spatial streams case.

The modulations are defined in tables 19-38 and 19-41 of the IEEE Std 802.11-2020.

Spotted by bz in https://reviews.freebsd.org/D39476
Sponsored by: Serenity Cybersecurity, LLC

[bz]

One note for both changes: you mention tables of the standard but do not say which version of the standard. You may want to add IEEE Std 802.11-XXXX with XXXX being the year of the standard you are referring to?

[misha]

In D39568#900712, @bz wrote:

One note for both changes: you mention tables of the standard but do not say which version of the standard. You may want to add IEEE Std 802.11-XXXX with XXXX being the year of the standard you are referring to?

Done, good point, thanks.