USB: reorganize code in hub.c

This patch (as1045) reorganizes some code in the hub driver.
hub_port_status() is moved earlier in the file, and a new hub_stop()
routine is created to do the work currently in hub_preset() (i.e.,
disconnect all child devices and quiesce the hub).

There are no functional changes.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>