USB: Add hub descriptor update hook for xHCI

Add a hook for updating xHCI internal structures after khubd fetches the
hub descriptor and sets up the hub's TT information.  The xHCI driver must
update the internal structures before devices under the hub can be
enumerated.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>