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>