ath9k: Report HT rates in RX status

Fix and clean up the RX status reporting by getting rid of code that
used internal rate tables and ratekbps calculation. The correct value
is now reported with MCS index instead of the old mechanism that
defaulted to using the highest legacy rate.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>