ath9k: remove ath9k_ht_macmode

This is used just to determine how to program the MAC,
either for 20 MHz operation of 40 MHz so just use conf_is_ht40()

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>