usb: gadget: Add Interface Association Descriptor to ECM

Add IAD to bind the two interfaces of ECM so that it works properly
in composite gadget mode.

Signed-off-by: Thirupathi <thirupathi.chippakurthy@stericsson.com>
Signed-off-by: Praveena Nadahally <praveen.nadahally@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>