ath9k: Fix TX status reporting for retries and MCS index

The count field in struct ieee80211_tx_rate does not include the final
successful attempt, so only report retries here.

Fix the struct ieee80211_tx_rate::idx field when MCS was used. It is
supposed to be the MCS index, not an internal index to the rate
control algorithm table.

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