cpufreq: tegra194: Rename tegra194_get_speed_common function

The function tegra194_get_speed_common() uses hardware timers to
calculate the current CPUFREQ and so rename this function to be
tegra194_calculate_speed() to reflect what it does.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>