rt2x00usb: Fix error return code

We know that 'retval = 0' because it has been tested a few lines above.
So, if 'devm_kmalloc' fails, 0 will be returned instead of an error code.
Return -ENOMEM instead.

Fixes: 8b4c0009313f ("rt2x00usb: Use usb anchor to manage URB")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/ralink/rt2x00/rt2x00usb.c

@@ -831,8 +831,10 @@ int rt2x00usb_probe(struct usb_interface *usb_intf,
 	rt2x00dev->anchor = devm_kmalloc(&usb_dev->dev,
 					sizeof(struct usb_anchor),
 					GFP_KERNEL);
-	if (!rt2x00dev->anchor)
+	if (!rt2x00dev->anchor) {
+		retval = -ENOMEM;
 		goto exit_free_reg;
+	}
 
 	init_usb_anchor(rt2x00dev->anchor);
 	return 0;