clk: tegra: Correct tegra210_pll_fixed_mdiv_cfg rate calculation

Return the actually achieved rate in cfg->output_rate rather than just
the requested rate. This is important to make clk_round_rate() return
the correct result.

Signed-off-by: Peter De Schrijver <pdeschrijver@nvidia.com>
Reviewed-by: Mikko Perttunen <mperttunen@nvidia.com>
Tested-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>