usb: dwc3: gadget: simplify next_request() return check

In dwc3_cleanup_done_reqs() we expect that all
iterations of our while (1) loop will find a valid
struct dwc3_request *. In case we don't, we're
dumping a WARN_ON_ONCE() splat so that people report
the failure.

This patch is a simple cleanup converting:

	if (!req) {
		WARN_ON_ONCE(1);
		return 1;
	}

to:

	if (WARN_ON_ONCE(!req))
		return 1;

which is a little easier to read.

Signed-off-by: Felipe Balbi <balbi@ti.com>