ath9k: Change rateset calculation

Commit "ath9k: Change rate control to use legacy rate as last MRR"
resulted in the mixing of HT/legacy rates in a single rateset,
which is undesirable. Revert this behavior.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>