mmc: meson-gx: rework clk_set function

Clean-up clk_set function to prepare the next changes (DDR and clk-stop)

Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>