spi/fsl-espi: make the clock computation easier to read

The -1 +1 thingy should probably do what DIV_ROUND_UP does. The 4 is 2
the  "platform_clock => sysclock" and 2 from the computation part. The 64
is the same 4 times 16.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

drivers/spi/spi-fsl-espi.c

@@ -180,7 +180,7 @@ static int fsl_espi_setup_transfer(struct spi_device *spi,
 
 	if ((mpc8xxx_spi->spibrg / hz) > 64) {
 		cs->hw_mode |= CSMODE_DIV16;
-		pm = (mpc8xxx_spi->spibrg - 1) / (hz * 64) + 1;
+		pm = DIV_ROUND_UP(mpc8xxx_spi->spibrg, hz * 16 * 4);
 
 		WARN_ONCE(pm > 16, "%s: Requested speed is too low: %d Hz. "
 			  "Will use %d Hz instead.\n", dev_name(&spi->dev),
@@ -188,7 +188,7 @@ static int fsl_espi_setup_transfer(struct spi_device *spi,
 		if (pm > 16)
 			pm = 16;
 	} else {
-		pm = (mpc8xxx_spi->spibrg - 1) / (hz * 4) + 1;
+		pm = DIV_ROUND_UP(mpc8xxx_spi->spibrg, hz * 4);
 	}
 	if (pm)
 		pm--;