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>