cpufreq: exynos: Initialize return variable

'ret' is undefined when the function returns from the first
'if' condition. Without this patch we get the following warning:

drivers/cpufreq/exynos-cpufreq.c: In function 'exynos_target':
drivers/cpufreq/exynos-cpufreq.c:182:2: warning: 'ret' may be used uninitialized in this function [-Wuninitialized]

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