usb_host: Do not unref USB devices when freeing the device list

If we tell libusb_free_device_list to unref the devices, we can end up in a
situation where our device handles are no longer valid, and we segfault. Do
not unref the devices, and figure out later how to not leak memory.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>