staging: emxx_udc: test returned value

Put NULL test on the result of the previous call instead on one of its
arguments.  A simplified version of the semantic match that finds this
problem is as follows (http://coccinelle.lip6.fr/):

// <smpl>
r@
expression *e1;
expression *e2;
identifier f;
statement S1,S2;
@@

e1 = f(...,e2,...);
(
if (e1 == NULL || ...) S1 else S2
|
*if (e2 == NULL || ...) S1 else S2
)
// </smpl>
Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/emxx_udc/emxx_udc.c

@@ -2998,7 +2998,7 @@ static void  nbu2ss_ep_fifo_flush(struct usb_ep *_ep)
 	}
 
 	ep = container_of(_ep, struct nbu2ss_ep, ep);
-	if (!_ep) {
+	if (!ep) {
 		pr_err("udc: %s, bad ep\n", __func__);
 		return;
 	}