editor's note: check if needed and reword or delete accordingly).
Returns the message instruction (b6-b1 of the message header) from the current
incomming HCI message, see TS 102 622 for further information.
This method gives the length of the received message data. Editor: is this length that is received or that is copied into the buffer? If the message is
not complete then the returned value is the actual message fragment length.