usb: Create link files between child device and usb port device.

To show the relationship between usb port and child device,
add link file "port" under usb device's sysfs directoy and
"device" under usb port device's sysfs directory. They are linked
to each other.

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