ath9k: apply coverage class on slottime too

According to 802.11-2007 17.3.8.6 (slot time), the slot time should
be increased by 3 us * coverage class. The code only increased the
ack timeout, which is fixed by this patch.

We have noticed in our long shot scenario that we see less collisions
with this patch.

Signed-off-by: Mathias Kretschmer <mathias.kretschmer@fokus.fraunhofer.de>
[add standard reference and commit message]
Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>