mmc: meson-gx: rework clock init function

Thanks to devm, carrying the clock structure around after init is not
necessary. Rework the function to remove these from the controller host
data.

Finally, set initial mmc clock rate before enabling it, simplifying the
exit condition.

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