clk: tegra: Return the exact clock rate from clk_round_rate

The current behavior is that clk_round_rate would return the same clock
rate passed to it for valid PLL configurations. This change will return
the exact rate the PLL will provide in accordance with clk API.

Signed-off-by: Robert Yang <decatf@gmail.com>
Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Tested-by: Dmitry Osipenko <digetx@gmail.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>