usb: gadget: rndis: fix Missing req->context assignment

It is crucial to assign each req->context value to struct rndis.

The problem happens for multi function gadget (g_multi) when multiple
functions are calling common usb_composite_dev control request.

It might happen that *_setup method from one usb function will
alter some fields of this common request issued by other USB
function.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>