ath9k_hw: fix the slot time setting for long distance links

Testing shows that adjusting the slot time based on the coverage class
produces very high latencies and very low throughput on long distance links.

Adjusting only the ACK timeout and leaving the slot time at the regular
values - while technically not optimal for CSMA - works a lot better on
long links (tested with 10 km distance)

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