staging: comedi: add comedi to usb interface helper

Add inline helper function `comedi_to_usb_interface()` to get the
pointer to `struct usb_interface` associated with the comedi device.
This pointer is set by the call to `comedi_usb_auto_config()`.
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/comedidev.h

@@ -41,6 +41,7 @@
 #include <linux/io.h>
 #include <linux/timer.h>
 #include <linux/pci.h>
+#include <linux/usb.h>
 
 #include "comedi.h"
 
@@ -193,8 +194,6 @@ struct comedi_async {
 			unsigned int x);
 };
 
-struct usb_interface;
-
 struct comedi_driver {
 	struct comedi_driver *next;
 
@@ -457,6 +456,12 @@ static inline struct pci_dev *comedi_to_pci_dev(struct comedi_device *dev)
 	return dev->hw_dev ? to_pci_dev(dev->hw_dev) : NULL;
 }
 
+static inline struct usb_interface *
+comedi_to_usb_interface(struct comedi_device *dev)
+{
+	return dev->hw_dev ? to_usb_interface(dev->hw_dev) : NULL;
+}
+
 int comedi_buf_put(struct comedi_async *async, short x);
 int comedi_buf_get(struct comedi_async *async, short *x);