ath9k_hw: fix a RTS/CTS timeout regression

commit adb5066a "ath9k_hw: do not apply the 2.4 ghz ack timeout
workaround to cts" reduced the hardware CTS timeout to the normal
values specified by the standard, but it turns out while it doesn't
need the same extra time that it needs for the ACK timeout, it
does need more than the value specified in the standard, but only
for 2.4 GHz.

This patch brings the CTS timeout value in sync with the initialization
values, while still allowing adjustment for bigger distances.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Cc: stable@vger.kernel.org
Reported-by: Seth Forshee <seth.forshee@canonical.com>
Reported-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>