usb: hub: add check for unsupported bus topology

We can't allow hubs on the 7th tier as they would allow
devices on the 8th tier.

Signed-off-by: Felipe Balbi <felipe.balbi@nokia.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>