drivers: spi_mcux_flexcomm: Fix spi slave count check

The current implementation checking for the validity of the spi slave
number is wrong. First of all, there is an off by one error where it
allows SPI_CHIP_SELECT_COUNT value as a valid slave, when really it
should be that value minus one. Secondly, it doesn't take into account
the fact that having software controlled GPIO CS can technically have
any number of slaves on the bus. So fix by finding the max of these two
values and fixing the off by one mistake. Also, for RW612, only one HW
native CS is available due to SOC pinmux limitations.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>

Author: Declan Snyder

drivers/spi/spi_mcux_flexcomm.c

@@ -1,6 +1,6 @@
 /*
  * Copyright (c) 2016, Freescale Semiconductor, Inc.
- * Copyright 2017, 2019, 2025, NXP
+ * Copyright 2017, 2019, 2025 NXP
  *
  * SPDX-License-Identifier: Apache-2.0
  */
@@ -29,7 +29,13 @@ LOG_MODULE_REGISTER(spi_mcux_flexcomm, CONFIG_SPI_LOG_LEVEL);
 
 #include "spi_context.h"
 
-#define SPI_CHIP_SELECT_COUNT	4
+#ifdef CONFIG_SOC_SERIES_RW6XX
+/* The RW61x SOC only allows use of HW chip select 0 */
+#define SPI_CHIP_SELECT_COUNT	1
+#else
+#define SPI_CHIP_SELECT_COUNT FSL_FEATURE_SPI_SSEL_COUNT
+#endif
+
 #define SPI_MAX_DATA_WIDTH	16
 #define SPI_MIN_DATA_WIDTH	4
 
@@ -225,9 +231,19 @@ static int spi_mcux_configure(const struct device *dev,
 			return -EINVAL;
 		}
 
-		if (spi_cfg->slave > SPI_CHIP_SELECT_COUNT) {
-			LOG_ERR("Slave %d is greater than %d",
-				    spi_cfg->slave, SPI_CHIP_SELECT_COUNT);
+		uint8_t max_slave = SPI_CHIP_SELECT_COUNT;
+
+#ifndef DT_SPI_CTX_HAS_NO_CS_GPIOS
+		struct spi_context *ctx = &data->ctx;
+
+		max_slave = MAX(max_slave, ctx->num_cs_gpios);
+#endif
+
+		max_slave -= 1;
+
+		if (spi_cfg->slave > max_slave) {
+			LOG_ERR("Slave %d is greater than max %d",
+				    spi_cfg->slave, max_slave);
 			return -EINVAL;
 		}