ot::Spinel::Buffer::OutFrameGetTag() method

Returns the tag value associated to current or front frame in the NCP frame buffer. If there is an active output from being read (an output frame was prepared earlier with successful call to `OutFrameBegin()`), this method returns the tag associated with current output frame. If there is no current active frame, the tag associated with the front frame in the queue (the next frame which would have been read) will be returned. If there is no frame in buffer, this method returns `kInvalidTag`.