staging: comedi: bug-fix NULL pointer dereference on failed attach

Commit dcd7b8bd63 ("staging: comedi: put
module _after_ detach" by myself) reversed a couple of calls in
`comedi_device_attach()` when recovering from an error returned by the
low-level driver's 'attach' handler.  Unfortunately, that introduced a
NULL pointer dereference bug as `dev->driver` is NULL after the call to
`comedi_device_detach()`.   We still have a pointer to the low-level
comedi driver structure in the `driv` variable, so use that instead.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Cc: <stable@vger.kernel.org> # 3.10+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers.c

@@ -482,7 +482,7 @@ int comedi_device_attach(struct comedi_device *dev, struct comedi_devconfig *it)
 		ret = comedi_device_postconfig(dev);
 	if (ret < 0) {
 		comedi_device_detach(dev);
-		module_put(dev->driver->module);
+		module_put(driv->module);
 	}
 	/* On success, the driver module count has been incremented. */
 	return ret;