cpufreq bug fix: variables should be surrouded by quotation marks.

Signed-off-by: hongbo.zhang <hongbo.zhang@linaro.com>
diff --git a/cpufreq/cpufreq_06.sh b/cpufreq/cpufreq_06.sh
index 215bf61..cb45e4b 100755
--- a/cpufreq/cpufreq_06.sh
+++ b/cpufreq/cpufreq_06.sh
@@ -107,7 +107,7 @@
 fi
 
 supported=$(cat $CPU_PATH/cpu0/cpufreq/scaling_available_governors | grep "userspace")
-if [ -z $supported ]; then
+if [ -z "$supported" ]; then
     log_skip "userspace not supported"
     exit 0
 fi