usb: dwc3: gadget: clear events in top-half handler

Now that all the infrastructure is in place, we can
clear events in the top-half handler in order to
bring IRQ line low ASAP.

This is also a necessary step in order to implement
workaround for known erratum in follow-up patches.

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