USB: fix __must_check warnings in drivers/usb/core/

Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/drivers/usb/core/usb.c b/drivers/usb/core/usb.c
index 6b029cd..60ef4ef 100644
--- a/drivers/usb/core/usb.c
+++ b/drivers/usb/core/usb.c
@@ -147,11 +147,13 @@
 struct usb_interface *usb_find_interface(struct usb_driver *drv, int minor)
 {
 	struct find_interface_arg argb;
+	int retval;
 
 	argb.minor = minor;
 	argb.interface = NULL;
-	driver_for_each_device(&drv->drvwrap.driver, NULL, &argb,
-			__find_interface);
+	/* eat the error, it will be in argb.interface */
+	retval = driver_for_each_device(&drv->drvwrap.driver, NULL, &argb,
+					__find_interface);
 	return argb.interface;
 }