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

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

diff --git a/drivers/usb/host/ehci-dbg.c b/drivers/usb/host/ehci-dbg.c
index 9cd6270..34b7a31 100644
--- a/drivers/usb/host/ehci-dbg.c
+++ b/drivers/usb/host/ehci-dbg.c
@@ -785,10 +785,11 @@
 static inline void create_debug_files (struct ehci_hcd *ehci)
 {
 	struct class_device *cldev = ehci_to_hcd(ehci)->self.class_dev;
+	int retval;
 
-	class_device_create_file(cldev, &class_device_attr_async);
-	class_device_create_file(cldev, &class_device_attr_periodic);
-	class_device_create_file(cldev, &class_device_attr_registers);
+	retval = class_device_create_file(cldev, &class_device_attr_async);
+	retval = class_device_create_file(cldev, &class_device_attr_periodic);
+	retval = class_device_create_file(cldev, &class_device_attr_registers);
 }
 
 static inline void remove_debug_files (struct ehci_hcd *ehci)