FOSSIL - initialize driver

        Entry:
        AH = 04h
        DX = port number
        optionally BX=4F50h
                   ES:CX -> byte to be set upon ^C
Return: AX = 1954h (if successful)
        BL = maximum function number supported (excluding 7Eh and above)
        BH = revision of FOSSIL supported
        DTR is raised
Note:   the word at offset 6 in the interrupt handler contains 1954h, and the
        following byte contains the maximum function number supported