ot::Spinel::Buffer::InFrameEnd() method

Finalizes/ends the current input frame being written to the buffer. Before using this method `InFrameBegin()` must be called to start and prepare a new input frame. Otherwise, this method does nothing and returns error status `OT_ERROR_INVALID_STATE`. If no buffer space is available, this method will discard and clear the frame and return error status `OT_ERROR_NO_BUFS`.