ath5k: configure backoff for IBSS beacon queue

in "11.1.2.2 Beacon generation in an IBSS" the IEEE802.11 standard says, each
STA should... "b) Calculate a random delay uniformly distributed in the range
between zero and twice aCWmin × aSlotTime,".

configure cwmin and cwmax of the beacon queue in IBSS mode according to this.
unfortunately beacon backoff does not work reliably yet, so i suspect we have a
problem somewhere else, since the same settings (and similar beacon timer
configuration) work for madwifi.

drivers/net/wireless/ath5k/base.c:      Changes-licensed-under: 3-Clause-BSD

Signed-off-by: Bruno Randolf <bruno@thinktube.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>