clocksource/drivers/tegra: Remove duplicated use of per_cpu_ptr

It was left unnoticed by accident, which means that the code could be
cleaned up a tad more.

Acked-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>