mmc: meson-gx: make sure the clock is rounded down

Using CLK_DIVIDER_ROUND_CLOSEST is unsafe as the mmc clock could be
rounded to a rate higher the specified rate. Removing this flag ensure
that, if the rate needs to be rounded, it will be rounded down.

Fixes: 51c5d844

 ("MMC: meson: initial support for GX platforms")
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>