usb: gadget: fix potential double-free in m66592_probe.

m66592_free_request() is called under label "err_add_udc"
and "clean_up", and m66592->ep0_req is not set to NULL after
first free, leading to a double-free. Fix this issue by
setting m66592->ep0_req to NULL after the first free.

Fixes: 0f91349b ("usb: gadget: convert all users to the new udc infrastructure")
Signed-off-by: Qiushi Wu <wu000273@umn.edu>
Signed-off-by: Felipe Balbi <balbi@kernel.org>

drivers/usb/gadget/udc/m66592-udc.c

@@ -1667,7 +1667,7 @@ static int m66592_probe(struct platform_device *pdev)
 
 err_add_udc:
 	m66592_free_request(&m66592->ep[0].ep, m66592->ep0_req);
-
+	m66592->ep0_req = NULL;
 clean_up3:
 	if (m66592->pdata->on_chip) {
 		clk_disable(m66592->clk);