usbnet: fix usb_autopm_get_interface failure(v1)

Since usbnet already took usb runtime pm, we have to
enable runtime pm for usb interface of usbnet, otherwise
usb_autopm_get_interface may return failure and cause
'ifconfig usb0 up' failed if USB_SUSPEND(RUNTIME_PM) is
enabled.

Cc: David Brownell <dbrownell@users.sourceforge.net>
Cc: Greg Kroah-Hartman <gregkh@suse.de>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Ben Hutchings <ben@decadent.org.uk>
Cc: Joe Perches <joe@perches.com>
Cc: Oliver Neukum <oliver@neukum.org>
Cc: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/usbnet.c

@@ -45,6 +45,7 @@
 #include <linux/usb/usbnet.h>
 #include <linux/slab.h>
 #include <linux/kernel.h>
+#include <linux/pm_runtime.h>
 
 #define DRIVER_VERSION		"22-Aug-2005"
 
@@ -1273,6 +1274,16 @@ usbnet_probe (struct usb_interface *udev, const struct usb_device_id *prod)
 	struct usb_device		*xdev;
 	int				status;
 	const char			*name;
+	struct usb_driver 	*driver = to_usb_driver(udev->dev.driver);
+
+	/* usbnet already took usb runtime pm, so have to enable the feature
+	 * for usb interface, otherwise usb_autopm_get_interface may return
+	 * failure if USB_SUSPEND(RUNTIME_PM) is enabled.
+	 */
+	if (!driver->supports_autosuspend) {
+		driver->supports_autosuspend = 1;
+		pm_runtime_enable(&udev->dev);
+	}
 
 	name = udev->dev.driver->name;
 	info = (struct driver_info *) prod->driver_info;