usb: dwc3: Do not traverse list using list_for_each_safe

There is a possibility of race between endpoint transfer
completion context and stop active transfer context. As
a result dwc3_gadget_ep_cleanup_completed_requests() iterates
over started_list using list_for_each_safe while
dwc3_remove_requests() is deleting req from started list.
While iterating over current list node list_for_each_safe caches
next list node. If the cached list node is getting deleted by
dwc3_remove_requests() then request for same list node would be
given back as part of endpoint transfer completion. This result
into list_del corruption because list node is getting deleted
twice. Fix this issue by iterating started_list using while loop
and always accessing first node from list head.

Change-Id: I64104a43ade015923deeb1f230a17dea08817052
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>