usb: dwc3: gadget: Protect against ep disabling during completion

In dwc3_cleanup_done_reqs(), a potential race condition
could arise when dwc3_gadget_giveback() temporarily
releases the main spinlock.  If during this window the
very endpoint being handled becomes disabled, it would
lead to a NULL pointer dereference in the code that
follows.  Guard against this by making sure the endpoint
is still enabled after returning from the giveback call.

CRs-fixed: 628972
Change-Id: Ifdb823fff12747f699217d871a5959c85b5340f7
Signed-off-by: Jack Pham <jackp@codeaurora.org>