$ grep PCI_CLASS_COMMUNICATION /usr/src/linux/include/linux/pci_ids.h 
#define PCI_CLASS_COMMUNICATION_SERIAL  0x0700
#define PCI_CLASS_COMMUNICATION_PARALLEL 0x0701
#define PCI_CLASS_COMMUNICATION_MULTISERIAL 0x0702
#define PCI_CLASS_COMMUNICATION_MODEM   0x0703
#define PCI_CLASS_COMMUNICATION_OTHER   0x0780

$ grep PCI_CLASS_COMMUNICATION
/usr/src/linux/drivers/serial/8250_pci.c 
        if ((((dev->class >> 8) != PCI_CLASS_COMMUNICATION_SERIAL) &&
             ((dev->class >> 8) != PCI_CLASS_COMMUNICATION_MODEM)) ||
                PCI_CLASS_COMMUNICATION_SERIAL << 8,
                PCI_CLASS_COMMUNICATION_MODEM << 8,
                PCI_CLASS_COMMUNICATION_MULTISERIAL << 8,

ungrab-winmodem.c