usb: dwc3: gadget: Discard events after cable disconnect

If controller is generating event after usb cable is unplugged
skip processing of events and simply acknowledge to hardware
how many events skipped.

CRs-Fixed: 747698
Change-Id: I19beb07cb0a98a92b691132d575f1ad76ab375e2
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>