dm: spi: Avoid setting the speed with every transfer

Only set the speed if it has changed from last time. Since the speed will
be 0 when the device is probed it will always be changed on the first
transfer after the device is probed.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/spi/spi-uclass.c b/drivers/spi/spi-uclass.c
index 83fe8e0..737ae64 100644
--- a/drivers/spi/spi-uclass.c
+++ b/drivers/spi/spi-uclass.c
@@ -63,9 +63,12 @@
 	}
 	if (!speed)
 		speed = 100000;
-	ret = spi_set_speed_mode(bus, speed, slave->mode);
-	if (ret)
-		return ret;
+	if (speed != slave->speed) {
+		ret = spi_set_speed_mode(bus, speed, slave->mode);
+		if (ret)
+			return ret;
+		slave->speed = speed;
+	}
 
 	return ops->claim_bus ? ops->claim_bus(dev) : 0;
 }