usb: gadget: composite: switch over to ERR_CAST()

This patch fixes the following Coccinelle warning:

drivers/usb/gadget/composite.c:1142:9-16: WARNING: \
	ERR_CAST can be used with uc
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/composite.c

@@ -1139,7 +1139,7 @@ struct usb_string *usb_gstrings_attach(struct usb_composite_dev *cdev,
 
 	uc = copy_gadget_strings(sp, n_gstrings, n_strings);
 	if (IS_ERR(uc))
-		return ERR_PTR(PTR_ERR(uc));
+		return ERR_CAST(uc);
 
 	n_gs = get_containers_gs(uc);
 	ret = usb_string_ids_tab(cdev, n_gs[0]->strings);