ath9k: Ensure a fair beacon distribution in IBSS mode.

Update the beacon queue parameters with best effort queue parameters for
IBSS mode. This reduces the number of beacons generated by ath9k and
ensures a fair beacon distribution when there are multiple IBSS stations.
Also CWmin is quadrupled to achieve the expected percentage of
distribution.

Signed-off-by: Vivek Natarajan <vnatarajan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>