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: NAdrian Hunter <adrian.hunter@intel.com>
Signed-off-by: NUlf Hansson <ulf.hansson@linaro.org>

drivers/mmc/core/core.c

@@ -995,7 +995,7 @@ void mmc_set_chip_select(struct mmc_host *host, int mode)
  */
 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;