spi/fsl-espi: Make sure pm is within 2..32

The reference manual says that pm has to stay within 2 and 32. So the
lowest frequency is 32 and DIV16 set, the highest is 2 and DIV16 unset.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

drivers/spi/spi-fsl-espi.c

@@ -182,16 +182,18 @@ static int fsl_espi_setup_transfer(struct spi_device *spi,
 		cs->hw_mode |= CSMODE_DIV16;
 		pm = DIV_ROUND_UP(mpc8xxx_spi->spibrg, hz * 16 * 4);
 
-		WARN_ONCE(pm > 16, "%s: Requested speed is too low: %d Hz. "
+		WARN_ONCE(pm > 33, "%s: Requested speed is too low: %d Hz. "
 			  "Will use %d Hz instead.\n", dev_name(&spi->dev),
-			  hz, mpc8xxx_spi->spibrg / 1024);
-		if (pm > 16)
-			pm = 16;
+				hz, mpc8xxx_spi->spibrg / (4 * 16 * (32 + 1)));
+		if (pm > 33)
+			pm = 33;
 	} else {
 		pm = DIV_ROUND_UP(mpc8xxx_spi->spibrg, hz * 4);
 	}
 	if (pm)
 		pm--;
+	if (pm < 2)
+		pm = 2;
 
 	cs->hw_mode |= CSMODE_PM(pm);