Uninitialized stack read of maxunequalmcs when HTC_TXUNEQUAL set but ic_txstream<2

Summary

ieee80211_setup_htrates(:1643) maxunequalmcs only assigned when ic_txstream>=2/3/4. If ic_txstream 0/1 with HTC_TXUNEQUAL set: indeterminate stack value used at :1678 to filter MCS 33-76. UB in kernel rate set computation. Driver config only.