Correct "interface" property on Linux hosts

[johannesacco]

The "interface" property should be read from the "usb_interface_path"
instead of the "device_path" as the later one is a directory (e.g.
/sys/devices/pci0000:00/0000:00:14.0/usb1/1-2/1-2.3/1-2.3:1.0/ttyUSB1).

This has been tested together with an CP2105EK where it is important
to separate the serial ports from each other.

[zsquareplusc]

can't reproduce with the composite device i have here (does not show an interface in any of the paths). Have you found some documentation on sys/.../interface?

[johannesacco]

I have tested with CP2105, CP2108, TTL232RG-VIP. All of them show "None" as interface without this PR.

With this PR the interface property is correctly initialized to match the one in the USB Descriptor (e.g. "lsusb -v -d 10c4:ea70 | grep iInterface", "lsusb -v -d 10c4:ea71 | grep iInterface" and "lsusb -v -d 0403:6001 | grep iInterface" for above devices.

@zsquareplusc which devices did you try out with and without this PR?

[zsquareplusc]

getting None on an STM that emulates 2 interfaces, getting duplicate text (same text for product and interface) on CP2104. but at least not None..

[johannesacco]

Thanks for merging!