INT 2F U - DOS 3+ internal - FLUSH BUFFER
        AX = 1215h
        DS:DI -> disk buffer
        SS = DOS DS (must be using a DOS internal stack)
        STACK: WORD drives for which to skip buffer
                ignore buffer if drive same as high byte, or bytes differ and
                the buffer is for a drive OTHER than that given in low byte
Return: STACK unchanged
Note:   can be called only from within DOS