ath5k: Implement mac80211 callback set_coverage_class

The callback sets slot time as specified in IEEE 802.11-2007 section
17.3.8.6 (for 20MHz channels only for now) and raises ACK and CTS
timeouts accordingly. The values are persistent, they are restored after
device reset.

Signed-off-by: Lukas Turek <8an@praha12.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>