ath9k: Enable HW queue control only for MCC

Enabling HW queue control for normal (non-mcc) mode
causes problems with queue management, resulting
in traffic stall. Since it is mainly required for
fairness in MCC mode, disable it for the general case.

Bug: https://dev.openwrt.org/ticket/18164

Cc: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>