usb-anchor: Ensure poisened gets initialized to 0

And do so in a way which ensures that any fields added in the future will
also get properly zero-ed.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Oliver Neukum <oliver@neukum.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>