ath9k: properly use the mac80211 rate control api

This patch changes ath9k to pass proper MCS indexes and flags
between the RC and the rest of the driver code.
sc->cur_rate_table remains, as it's used by the RC code internally,
but the rest of the driver code no longer uses it, so a potential
new RC for ath9k would not have to update it.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>