clk: meson: honor CLK_MUX_ROUND_CLOSEST in clk_regmap

Using __clk_mux_determine_rate effectively ignores CLK_MUX_ROUND_CLOSEST
if set the related clk_regmap mux instance.

Use clk_mux_determine_rate_flags() to make sure the flag is honored.

Fixes: ea11dda9 ("clk: meson: add regmap clocks")
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>