ath9k: Update rate control for 11NA HT40 mode

Now the lowest rate in 11na ht40 mode is 13.5Mbps this shortens the
range when compared to 11na ht20 mode where the lowest rate is 6.5Mbps.
To improve the range, make 6.5Mbps as the lowest rate in 11na ht40 mode,
this improves the range by approximately 2dB. 11ng ht40 does not have
this issue as it also has basic rates (1, 2, 5.5 and 11).

Signed-off-by: Vasanthakumar Thiagarajan <vasanth@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>