PCI serial devices missing from available_ports() on Linux without libudev

[sirhcel]

When checking #220, I noticed that the two ports from a PCI serial card are missing. With libudev, there are two more PCI serial ports reported than without:

$ cargo run --features usbportinfo-interface --example list_ports
[...]
Found 10 ports:
    /dev/ttyS0
        Type: Unknown
    /dev/ttyS4
        Type: PCI
    /dev/ttyS5
        Type: PCI
    /dev/ttyUSB0
        Type: USB

$ cargo run --no-default-features --features usbportinfo-interface --example list_ports
[...]
Found 8 ports:
    /dev/ttyS0
        Type: Unknown
    /dev/ttyUSB0
        Type: USB

Checked with 944fe82.