spi: dw: Set default value if reg-io-width isn't specified

According to the dt-bindings, the default value of reg-io-width is 4.
However, the value becomes zero when reg-io-width isn't specified.

Should set the actual value to dws->reg_io_width, considering it
referenced.

Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Link: https://lore.kernel.org/r/20230807001621.196776-1-hayashi.kunihiko@socionext.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: khayash1

drivers/spi/spi-dw-mmio.c

@@ -369,7 +369,9 @@ static int dw_spi_mmio_probe(struct platform_device *pdev)
 
 	dws->max_freq = clk_get_rate(dwsmmio->clk);
 
-	device_property_read_u32(&pdev->dev, "reg-io-width", &dws->reg_io_width);
+	if (device_property_read_u32(&pdev->dev, "reg-io-width",
+				     &dws->reg_io_width))
+		dws->reg_io_width = 4;
 
 	num_cs = 4;