usb: move children deallocation after quiescing the hub

Commit ff823c79 ("usb: move children
to struct usb_port") forgot to consider the hub_disconnect sequence,
which releases ports before quiescing the hub, which will lead to a
use-after-free, since hub_quiesce() will try to disconnect ports'
children, which are already deallocated. Simple modprobe dummy_hcd &&
rmmod dummy_hcd will illustrate the problem.

This patch moves deallocation of hub's ports after hub_quiesce() call
in hub_disconnect().

Cc: Lan Tianyu <tianyu.lan@intel.com>
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>