memory: tegra: Correct debugfs clk rate-range on Tegra20

Correctly set clk rate-range if number of available timings is zero.
This fixes noisy "invalid range [4294967295, 0]" error messages during
boot.

Fixes: 8209eefa ("memory: tegra: Implement EMC debugfs interface on Tegra20")
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>