rust: usb: keep usb::Device private for now

rust: usb: keep usb::Device private for now

The USB abstractions target to support USB interface drivers.

While internally the abstraction has to deal with the interface's parent
USB device, there shouldn't be a need for users to deal with the parent
USB device directly.

Functions, such as for preparing and sending USB URBs, can be
implemented for the usb::Interface structure directly. Whether this
internal implementation has to deal with the parent USB device can
remain transparent to USB interface drivers.

Hence, keep the usb::Device structure private for now, in order to avoid
confusion for users and to make it less likely to accidentally expose
APIs with unnecessary indirections.

Should we start supporting USB device drivers, or need it for any other
reason we do not foresee yet, it should be trivial to make it public
again.

Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20250925190400.144699-2-dakr@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>