clk: tegra: Fix clock rate computation

clk: tegra: Fix clock rate computation

The PLL output frequency is multiplied during the P-divider computation,
so it needs to be divided by the P-divider again before returning.

This fixes an issue where clk_round_rate() would return the multiplied
frequency instead of the real one after the P-divider.

Signed-off-by: Thierry Reding <treding@nvidia.com>