INT 7A - Novell NetWare - IPX Driver - CANCEL EVENT
        BX = 0006h
        ES:SI -> Event Control Block (see BX=0003h)
Return: AL = return code
            00h success
            F9h event in use
            FCh event cancelled
            FFh unsuccessful, event not in use
Notes:  cannot cancel packets which the node's driver has already sent
        this function is supported by Advanced NetWare 1.02+
SeeAlso: BX=0005h