in 2.4 ghz, calculating the ack timeout the way it is described in 802.11-2007 currently leads to tx timeouts
set it to a minimum of what the initvals set (64 usec) until we know what is causing this
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@19156 3c298f89-4303-0410-b956-a3cf2f4a3e73