spi_mpc83xx: quieten down the "Requested speed is too low" message

When a platform is running at high frequencies it's not always possible to
scale-down a frequency to a requested value, and using mmc_spi driver this
leads to the following printk flood during card polling:

  ...
  mmc_spi spi32766.0: Requested speed is too low: 400000 Hz. Will use
  520828 Hz instead.
  mmc_spi spi32766.0: Requested speed is too low: 400000 Hz. Will use
  520828 Hz instead.
  ...

Fix this by using WARN_ONCE(), it's better than the flood, and also better
than turning dev_err() into dev_dbg(), since we actually want to warn that
some things may not work correctly.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Cc: Kumar Gala <galak@gate.crashing.org>
Cc: David Brownell <david-b@pacbell.net>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/spi/spi_mpc83xx.c

@@ -14,6 +14,7 @@
 #include <linux/init.h>
 #include <linux/types.h>
 #include <linux/kernel.h>
+#include <linux/bug.h>
 #include <linux/errno.h>
 #include <linux/err.h>
 #include <linux/completion.h>
@@ -275,12 +276,12 @@ int mpc83xx_spi_setup_transfer(struct spi_device *spi, struct spi_transfer *t)
 	if ((mpc83xx_spi->spibrg / hz) > 64) {
 		cs->hw_mode |= SPMODE_DIV16;
 		pm = mpc83xx_spi->spibrg / (hz * 64);
-		if (pm > 16) {
-			dev_err(&spi->dev, "Requested speed is too "
-				"low: %d Hz. Will use %d Hz instead.\n",
-				hz, mpc83xx_spi->spibrg / 1024);
+
+		WARN_ONCE(pm > 16, "%s: Requested speed is too low: %d Hz. "
+			  "Will use %d Hz instead.\n", dev_name(&spi->dev),
+			  hz, mpc83xx_spi->spibrg / 1024);
+		if (pm > 16)
 			pm = 16;
-		}
 	} else
 		pm = mpc83xx_spi->spibrg / (hz * 4);
 	if (pm)