cpufreq: ia64: don't set .driver_data to index

.driver_data field is only required to be filled if drivers want to
preserve some data in there which they can use according to the value
of .frequency. But this driver isn't using this field at all, but just
setting it equal to the index value. Which isn't required. Fix it.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>