usb/core: Added devspec sysfs entry for devices behind the usb hub

Grub finds incorrect of_node path for devices behind usb hub.
Added devspec sysfs entry for devices behind usb hub so that
right of_node path is returned during grub sysfs walk for these
devices.

Signed-off-by: Vijay Kumar <vijay.ac.kumar@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>