madwifi: rework tx power handling, don't clamp user-configured tx power limit...

madwifi: rework tx power handling, don't clamp user-configured tx power limit based on current channel properties

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@15031 3c298f89-4303-0410-b956-a3cf2f4a3e73