usb: dwc3: gadget: combine return points into a single one

dwc3_send_gadget_ep_cmd() had three return
points. That becomes a pain to track when we need to
debug something or if we need to add more code
before returning.

Let's combine all three return points into a single
one just by introducing a local 'ret' variable.

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>