trivial: Partially revert some of 464eacdc
trivial: Partially revert some of 464eacdc

The FuUsbDevice was using a locker of NULL to indicate unlocked, and just
calling `fu_device_locker_close()` does not unset the object.

hughsie/fwupd,fwupd/fwupd,hughsie/fwupd,hughsie/fwupd,hughsie/fwupd,fwupd/fwupd,fwupd/fwupd,fwupd/fwupd