spi: core: use gpio_is_valid() helper

Check if GPIO pin is valid by API helper function.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi.c

@@ -606,7 +606,7 @@ static void spi_set_cs(struct spi_device *spi, bool enable)
 	if (spi->mode & SPI_CS_HIGH)
 		enable = !enable;
 
-	if (spi->cs_gpio >= 0)
+	if (gpio_is_valid(spi->cs_gpio))
 		gpio_set_value(spi->cs_gpio, !enable);
 	else if (spi->master->set_cs)
 		spi->master->set_cs(spi, !enable);