The devc-ser8250 driver should automatically detect the modem, which should appear in the /dev directory as serx, where x is an integer.
If no entry is created, check the output from pci -vvv and see what I/O port and IRQ are assigned to the modem. Use the correct I/O port and IRQ from pci -vvv to start devc-ser8250. When you use the appropriate I/O port and IRQ, the /dev directory entry gets created for you.