ath9k_hw: increase ACK timeout for half/quarter channels

For some reason the MAC timing is a bit off when waiting for ACKs, so add
some extra delay to the ACK timeout values. Significantly reduces the
number of retransmissions in my tests.
Also disable the 2.4 GHz ACK timeout workaround in half/quarter mode, it
is not required there.

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