ath9k: Ensure a fair beacon distribution in IBSS mode

Configure CWmin based on slot time for IBSS mode. This helps
in increasing the beacon distribution of ath9k to accepted
levels in 11a mode.

Signed-off-by: Vivek Natarajan <nataraja@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>