cpufreq: exynos: Fix unsigned variable being checked for negative value

exynos_cpufreq_scale function returns signed value which was
assigned to an unsigned variable and checked for negative value which
is always false. Hence make it signed.

Fixes the following smatch warnings:
drivers/cpufreq/exynos-cpufreq.c:83 exynos_cpufreq_scale() warn: unsigned 'old_index' is never less than zero.
drivers/cpufreq/exynos-cpufreq.c:89 exynos_cpufreq_scale() warn: unsigned 'index' is never less than zero.
Signed-off-by: NSachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: NKukjin Kim <kgene.kim@samsung.com>

drivers/cpufreq/exynos-cpufreq.c

@@ -65,7 +65,7 @@ static int exynos_cpufreq_scale(unsigned int target_freq)
 	struct cpufreq_policy *policy = cpufreq_cpu_get(0);
 	unsigned int arm_volt, safe_arm_volt = 0;
 	unsigned int mpll_freq_khz = exynos_info->mpll_freq_khz;
-	unsigned int index, old_index;
+	int index, old_index;
 	int ret = 0;
 
 	freqs.old = policy->cur;