During a routine audit of udev scripts in openSUSE Factory, it was noticed that
libiio-usb-udev-rules grants world-writable permissions:

```
SUBSYSTEM=="usb", PROGRAM=="/bin/sh -c '/usr/bin/iio_info -S
usb=%s{idVendor}:%s{idProduct} | grep %s{idVendor}:%s{idProduct}'", RESULT!="",
MODE="666"
```

This means unprivileged users have unrestricted read/write access to any IIO
USB device or any device deemed by `iio_info` to be an IIO device.

While this is not a security vulnerability per se, yet, hardening measures are
warranted. It would be preferable to restrict access to a group, e.g.:
MODE="660", GROUP="plugdev", or at least MODE="0664", GROUP="plugdev".

What do you think?

For reference, we already have udev rules with stricter permissions:
```
i+ | rtl-sdr-udev | Udev rules for RTL2832 | package
i+ | uhd-udev | UHD udev rules | package
```