BOOLEAN
CpDoesPortExist(
  IN  PUCHAR Address
  );

Routine Description:

    This routine will attempt to place the port into its
    diagnostic mode.  If it does it will twiddle a bit in
    the modem control register.  If the port exists this
    twiddling should show up in the modem status register.

    NOTE: This routine must be called before the device is
          enabled for interrupts, this includes setting the
          output2 bit in the modem control register.

    This is blatantly stolen from TonyE's code in ntos\dd\serial\serial.c.

Arguments:

    Address - address of hw port.

Return Value:

    TRUE - Port exists.

    FALSE - Port doesn't exist.