mfd: tps6594: Use spi_get_chipselect() API to access spi->chip_select

In preparation for adding multiple CS support for a device, set/get
functions were introduces accessing spi->chip_select in
'commit 303feb3c ("spi: Add APIs in spi core to set/get
spi->chip_select and spi->cs_gpiod")'.
Replace spi->chip_select with spi_get_chipselect() API.
Signed-off-by: Amit Kumar Mahapatra <amit.kumar-mahapatra@amd.com>
Link: https://lore.kernel.org/r/20231125092137.2948-2-amit.kumar-mahapatra@amd.comSigned-off-by: Lee Jones <lee@kernel.org>

drivers/mfd/tps6594-spi.c

@@ -98,7 +98,7 @@ static int tps6594_spi_probe(struct spi_device *spi)
 	spi_set_drvdata(spi, tps);
 
 	tps->dev = dev;
-	tps->reg = spi->chip_select;
+	tps->reg = spi_get_chipselect(spi, 0);
 	tps->irq = spi->irq;
 
 	tps->regmap = devm_regmap_init(dev, NULL, spi, &tps6594_spi_regmap_config);