Actuator group hotfix

@LorenzMeier, This is hotfix for actuator controls group "manual passthrough". Documentation (http://pixhawk.org/dev/mixing) says that it should be on group 3, but PX4IO supports only two groups (https://github.com/PX4/Firmware/blob/master/src/modules/px4iofirmware/px4io.h#L56) and group 3 ignored, moreover I'm not sure that this code will not overwrite something useful:
https://github.com/PX4/Firmware/blob/master/src/drivers/px4io/px4io.cpp#L978
https://github.com/PX4/Firmware/blob/master/src/drivers/px4io/px4io.cpp#L1038

This is only hotfix to get AUX outputs working, but need to find right solution. If we really want 4 actuator groups, support for it can be added to PX4IO. But my rule is "think about flexibility of code and API, but don't add unused features". So maybe just rewrite documentation and use only two actuator groups? When we will need gimbal and payload and we will have apps for this, we can add new actuator groups.