Modify the TX path to set and use the 11n rate scenario bits.
Modify the TX path to set and use the 11n rate scenario bits.

This isn't strictly required to TX (at least non-agg and non-HT40,
non-short-GI) frames; but as it needs to be done anyway, just get
it done.

Linux ath9k uses the rate scenario style path for -all- packets,
legacy or otherwise. This code does much the same.

Beacon TX still uses the legacy, non-rate-scenario TX descriptor
setup. Ath9k also does this.

This 11n rate scenario path is only called for chips in the AR5416
HAL; legacy chips use the previous interface for TX'ing.

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