ath9k: add a workaround for ack timeout issues

Adding support for setting the coverage class in some cases broke
association and data transfer, as it overwrote the initial ACK timeout
value from the initvals with a smaller value.
I don't know why the new value works in 5 GHz (matches the initval
there), but not in 2.4 GHz (initvals use 64us here), so until the
problem is fully understood, the value should be increased again.

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