mmc: Fix incorrect warning when setting 0 Hz via debugfs

It is possible to turn off the card clock by setting
the frequency to zero via debugfs e.g.

	echo 0 > /sys/kernel/debug/mmc0/clock

However that produces an incorrect warning that is
designed to warn if the frequency is below the minimum
operating frequency.  So correct the warning.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
diff --git a/drivers/mmc/core/core.c b/drivers/mmc/core/core.c
index cbb2321..f26a5f1 100644
--- a/drivers/mmc/core/core.c
+++ b/drivers/mmc/core/core.c
@@ -995,7 +995,7 @@
  */
 static void __mmc_set_clock(struct mmc_host *host, unsigned int hz)
 {
-	WARN_ON(hz < host->f_min);
+	WARN_ON(hz && hz < host->f_min);
 
 	if (hz > host->f_max)
 		hz = host->f_max;