staging: comedi: s626: add mmiowb() calls

`s626_mc_enable()` is often called to enable some function in a control
register after setting up some other registers.  Precede the write to
the control register with a call to `mmiowb()` to preserve mmio write
ordering.

`s626_mc_disable()` is called to disable some function in a control
register and is often followed up by writes to other registers.  Follow
the write to the control register with a call to `mmiowb()` to preserve
mmio write ordering.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/s626.c

@@ -161,6 +161,7 @@ static void s626_mc_enable(struct comedi_device *dev,
 	struct s626_private *devpriv = dev->private;
 	unsigned int val = (cmd << 16) | cmd;
 
+	mmiowb();
 	writel(val, devpriv->mmio + reg);
 }
 
@@ -170,6 +171,7 @@ static void s626_mc_disable(struct comedi_device *dev,
 	struct s626_private *devpriv = dev->private;
 
 	writel(cmd << 16 , devpriv->mmio + reg);
+	mmiowb();
 }
 
 static bool s626_mc_test(struct comedi_device *dev,