ath9k_hw: Disable PAPRD for rates with low Tx power

When the drop in Tx power for a particular mcs rate exceeds
the paprd scale factor, paprd may not work properly. Disable
paprd for any such rates.

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