ath9k_htc: implement sta_rc_update() mac80211 callback

In case of changes in the supported rates set for a given station, it is now
possible to use this callback to update the current internal state of the
station in the htc driver.

Reported-by: Gui Iribarren <gui@altermundi.net>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>