ath9k: remove sc->cur_rate_table and sc->cur_rate_mode

Set the rate table in the rc module properly based on band and
HT capabilities instead, which was already partially done, but
not for every mode.

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