mmc: meson-gx: fix error path in meson_mmc_clk_init / meson_mmc_probe

The condition should be "if (ret)" as the disable/unprepare is
supposed to be executed if the previous command fails.
In addition adjust the error path in probe to properly deal
with the case that cfg_div_clk can be registered successfully
but enable/prepare fails.
In this case we shouldn't call clk_disable_unprepare.

Reported-by: Michał Zegan <webczat_200@poczta.onet.pl>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Acked-by: Kevin Hilman <khilman@baylibre.com>
Tested-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>