usb: gadget: ecm: Add IAD descriptor in SS mode

Commit d11519 ("usb: gadget: Add Interface Association Descriptor to
ECM") added the IAD descriptor to FS and HS descriptors. The SS
descriptor has been left out probably because it has been added "just
recently".

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>