usb: dwc3: gadget: Refactor dwc3_gadget_ep_dequeue

The flow from function dwc3_gadget_ep_dequeue() is not easy to follow.
Refactor it for easier read. No functional change in this commit.

Signed-off-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>