ath5k: fix slot time handling

Set the slot time based on the mac80211 short slot vs long slot setting
instead of just forcing long slot for all CCK-enabled channels.
This slightly improves 802.11g mode performance in in my tests.

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