There is no way (that I know of) that allows you to go directly from the serial port on the PC to the MPI port on an S7-300.
Thinking of it, it would require a driver to handle the MPI protocol directly in software.
Not impossible, but there is no such thing available at the moment.
The "SoftNet" drivers for CP5511 and CP5611 are not 100% software based (as far as I know).