cpufreq: s3c2416: fix forgotten driver_data conversions

Commit 50701588 (cpufreq: rename index as driver_data in
cpufreq_frequency_table) renamed the index field to driver_data.

But it seems some uses in the s3c2416 driver were forgotten.

So convert the last index users to read driver_data.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>