USB: fix PTR_ERR translation in init_usb_class()

There is a misprint in init_usb_class():
IS_ERR is used to get error code instead of PTR_ERR.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/core/file.c

@@ -94,7 +94,7 @@ static int init_usb_class(void)
 	kref_init(&usb_class->kref);
 	usb_class->class = class_create(THIS_MODULE, "usbmisc");
 	if (IS_ERR(usb_class->class)) {
-		result = IS_ERR(usb_class->class);
+		result = PTR_ERR(usb_class->class);
 		printk(KERN_ERR "class_create failed for usb devices\n");
 		kfree(usb_class);
 		usb_class = NULL;