regulator: tps6586x: Use dev_err rather than dev_warn for error message

tps6586x_regulator_set_slew_rate() returns -EINVAL when having slew rate
settings for other than SM0/1, thus use dev_err rather than dev_warn.
Signed-off-by: Axel Lin <axel.lin@ingics.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

drivers/regulator/tps6586x-regulator.c

@@ -245,7 +245,7 @@ static int tps6586x_regulator_set_slew_rate(struct platform_device *pdev,
 		reg = TPS6586X_SM1SL;
 		break;
 	default:
-		dev_warn(&pdev->dev, "Only SM0/SM1 can set slew rate\n");
+		dev_err(&pdev->dev, "Only SM0/SM1 can set slew rate\n");
 		return -EINVAL;
 	}