ath9k: Change rate control to use legacy rate as last MRR

In congested network, having all rate reties at MCS rates
is failing to transmit the frame offenly. By the time reaching
the success rate set, the application gets timed out. One such
scenario is that authentication time out during 4-Way handshake.
This patch uses a legacy rate as last retry sequnce for
unaggregated frames or if the first selected rate's PER is ~80%
of max limit. And also observed from the tx status that the frame
was trasmitted successfully by using legacy rates.

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