dma: mv_xor: Flush descriptors before activating a channel

We need to use writel() instead of writel_relaxed() when starting
a channel, to ensure all the descriptors have been flushed before
the activation.

While at it, remove the unneeded read-modify-write and make the
code simpler.

Cc: <stable@vger.kernel.org>
Signed-off-by: Lior Amsalem <alior@marvell.com>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/dma/mv_xor.c

@@ -191,12 +191,10 @@ static void mv_set_mode(struct mv_xor_chan *chan,
 
 static void mv_chan_activate(struct mv_xor_chan *chan)
 {
-	u32 activation;
-
 	dev_dbg(mv_chan_to_devp(chan), " activate chan.\n");
-	activation = readl_relaxed(XOR_ACTIVATION(chan));
-	activation |= 0x1;
-	writel_relaxed(activation, XOR_ACTIVATION(chan));
+
+	/* writel ensures all descriptors are flushed before activation */
+	writel(BIT(0), XOR_ACTIVATION(chan));
 }
 
 static char mv_chan_is_busy(struct mv_xor_chan *chan)