ugen: fix USB_IFACE_DRIVER_ACTIVE after detaching a driver

Previosuly, USB_IFACE_DRIVER_ACTIVE would report that the driver is
active even after it detached. That's because a device(9) still
remains.

So, add device_is_alive(9) check for more accurate reporting.

Reviewed by: markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D43960