USB: dwc3: gadget.c: Check ep0 request list is empty or not

There is chance for disconnect processing and control endpoint
completion handler run in parallel. As part of disconnect,
control endpoint's request list will be dequeued and list becomes
empty. After this, if completion handler for control endpoint is
executed, it might lead to crash  as driver is not checking
whether request list  is empty or not. Hence check whether request
list for control endpoint is empty in completion handler.

CRs-Fixed: 481554
Change-Id: I686d727c1686267707f43d94c57607c3783c7d90
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>