clk: rockchip: Use clk_hw_get_rate() in MMC phase calculation

When calculating the MMC phase we can just use clk_hw_get_rate()
instead of clk_get_rate().  This avoids recalculating the rate.

Suggested-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>