avoid static mut for USB test device builder

[robamu]

clippy is not happy about static muts anymore.

For some reason, the tests do not run for me:

test_class_host starting
looking for device...
Did not find a TestClass device. Make sure the device is correctly programmed and plugged in. Last error: No such device (it may have been disconnected)
   Doc-tests usb_device

I installed the rules file, so not sure what is going on..