spi: spi-fsl-lpspi: Treat prescale_max == 0 as no erratum

This erratum only ever results in a max value of 1, otherwise the full 3
bits are available. To avoid repeating the same default prescale value
for every new device's devdata, treat 0 as no limit (7) and only set a
value when the erratum is present.

Change the field to be 3 bits to catch out of range definitions.

No functionality change.

Signed-off-by: James Clark <james.clark@linaro.org>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Link: https://patch.msgid.link/20250828-james-nxp-lpspi-v2-7-6262b9aa9be4@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: james-c-linaro

drivers/spi/spi-fsl-lpspi.c

@@ -86,7 +86,7 @@
 #define SR_CLEAR_MASK	GENMASK(13, 8)
 
 struct fsl_lpspi_devtype_data {
-	u8 prescale_max;
+	u8 prescale_max : 3; /* 0 == no limit */
 };
 
 struct lpspi_config {
@@ -132,15 +132,15 @@ struct fsl_lpspi_data {
 };
 
 /*
- * ERR051608 fixed or not:
- * https://www.nxp.com/docs/en/errata/i.MX93_1P87f.pdf
+ * Devices with ERR051608 have a max TCR_PRESCALE value of 1, otherwise there is
+ * no prescale limit: https://www.nxp.com/docs/en/errata/i.MX93_1P87f.pdf
  */
 static const struct fsl_lpspi_devtype_data imx93_lpspi_devtype_data = {
 	.prescale_max = 1,
 };
 
 static const struct fsl_lpspi_devtype_data imx7ulp_lpspi_devtype_data = {
-	.prescale_max = 7,
+	/* All defaults */
 };
 
 static const struct of_device_id fsl_lpspi_dt_ids[] = {
@@ -324,7 +324,7 @@ static int fsl_lpspi_set_bitrate(struct fsl_lpspi_data *fsl_lpspi)
 	int scldiv;
 
 	perclk_rate = clk_get_rate(fsl_lpspi->clk_per);
-	prescale_max = fsl_lpspi->devtype_data->prescale_max;
+	prescale_max = fsl_lpspi->devtype_data->prescale_max ?: 7;
 
 	if (!config.speed_hz) {
 		dev_err(fsl_lpspi->dev,