staging: usbip: userspace: don't throw error when trying to read configuration specific attributes

When a device has just been bound to usbip-host but the client hasn't
set a configuration on it, certain attributes will not exist. Don't
treat this as an error.

Signed-off-by: Valentina Manea <valentina.manea.m@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>