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: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>