staging: usbip: Notify usb core of port status changes

This patch changes vhci to behave like dummy and
other hcds when disconnecting a device.

Previously detaching a device from the root hub
did not notify the usb core of the disconnect and
left the device visible.

Signed-off-by: Max Vozeler <mvz@vozeler.com>
Reported-by: Marco Lancione <marco@optikam.com>
Tested-by: Luc Jalbert <ljalbert@optikam.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>