usbip: vudc: make dev_desc attribute binary

We read a struct usb_device_descriptor from it, so make it an actual
binary attribute.

Signed-off-by: Igor Kotrasinski <i.kotrasinsk@gmail.com>
Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Tested-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>