usb: gadget: Fix OS descriptors support

The current code is broken as it re-defines "req" inside the
if block, then goto out of it. Thus the request that ends
up being sent is not the one that was populated by the
code in question.

This fixes RNDIS driver autodetect by Windows 10 for me.

The bug was introduced by Chris rework to remove the local
queuing inside the if { } block of the redefined request.

Fixes: 636ba13a ("usb: gadget: composite: remove duplicated code in OS desc handling")
Cc: <stable@vger.kernel.org> # v4.17
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>