Make sure the USB device pointer is valid even if attach fails since it is now checked to prevent any operation on the device.
Make sure the USB device pointer is valid even if attach fails since
it is now checked to prevent any operation on the device.

Problem found and fix tested by sebastia@

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars