rust: usb: don't retain device context for the interface parent

rust: usb: don't retain device context for the interface parent

When deriving the parent USB device (struct usb_device) from a USB
interface (struct usb_interface), do not retain the device context.

For the Bound context, as pointed out by Alan in [1], it is not
guaranteed that the parent USB device is always bound when the interface
is bound.

The bigger problem, however, is that we can't infer the Core context,
since eventually it indicates that the device lock is held. However,
there is no guarantee that if the device lock of the interface is held,
also the device lock of the parent USB device is held.

Hence, fix this by not inferring any device context information; while
at it, fix up the (affected) safety comments.

Link: https://lore.kernel.org/all/0ff2a825-1115-426a-a6f9-df544cd0c5fc@rowland.harvard.edu/
Fixes: e7e2296b0ecf ("rust: usb: add basic USB abstractions")
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20250925190400.144699-1-dakr@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>