ath9k: choose legacy rate as last rate of MRR series

Choose legacy rate as the last rate of Multi Rate Retry series
if and only if the last selected rate is MCS and having higher
PER rate. The current code fills a legacy rate as last one even
though the previous rates in the series are having good PER value.
This could limit the aggregation that affects the uplink performance.

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