staging: comedi: amplc_pc236: fix invalid register access during detach

`pc236_detach()` is called by the comedi core if it attempted to attach
a device and failed.  `pc236_detach()` calls `pc236_intr_disable()` if
the comedi device private data pointer (`devpriv`) is non-null.  This
test is insufficient as `pc236_intr_disable()` accesses hardware
registers and the attach routine may have failed before it has saved
their I/O base addresses.

Fix it by checking `dev->iobase` is non-zero before calling
`pc236_intr_disable()` as that means the I/O base addresses have been
saved and the hardware registers can be accessed.  It also implies the
comedi device private data pointer is valid, so there is no need to
check it.

Cc: <stable@vger.kernel.org> # 3.5.x, 3.6.x
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/amplc_pc236.c

@@ -573,9 +573,8 @@ static int __devinit pc236_attach_pci(struct comedi_device *dev,
 static void pc236_detach(struct comedi_device *dev)
 {
 	const struct pc236_board *thisboard = comedi_board(dev);
-	struct pc236_private *devpriv = dev->private;
 
-	if (devpriv)
+	if (dev->iobase)
 		pc236_intr_disable(dev);
 	if (dev->irq)
 		free_irq(dev->irq, dev);