ath9k_hw: do not apply the 2.4 ghz ack timeout workaround to cts

It is only used to workaround interoperability issues related to longer
delays in receiving the block ack, so it is not necessary to apply it
to the CTS exchange.
Should improve throughput slightly, especially when there are lots
of retransmissions.

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