usb: gadget: udc: fsl: Add missing complete function.

Commit 304f7e5e ("usb: gadget: Refactor request completion")
removed check if req->req.complete is non-NULL, resulting in a NULL
pointer derefence and a kernel panic.
This patch adds an empty complete function instead of re-introducing
the req->req.complete check.

Fixes: 304f7e5e ("usb: gadget: Refactor request completion")

Signed-off-by: Magnus Lilja <lilja.magnus@gmail.com>
Cc: <stable@vger.kernel.org> # 3.18+
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>