staging: comedi: ni_660x: local functions should not be exported

The function ni_660x_release_mite_channel() is only referenced in
this file. Make it static.
Signed-off-by: NH Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/ni_660x.c

@@ -865,8 +865,8 @@ static int ni_660x_request_mite_channel(struct comedi_device *dev,
 	return 0;
 }
 
-void ni_660x_release_mite_channel(struct comedi_device *dev,
-				  struct ni_gpct *counter)
+static void ni_660x_release_mite_channel(struct comedi_device *dev,
+					 struct ni_gpct *counter)
 {
 	unsigned long flags;