usb: dwc3: gadget: use evt->cache for processing events

Let's start copying events from evt->buf to
evt->cache and use evt->cache for processing events.

A follow-up patch will be added to clear events in
the top-half handler in order to bring IRQ line low
as soon as possible.

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