mmc: meson-gx: fix error path when driver initialization fails before enabling clocks

This patch fixes the case where meson_mmc_probe function fails before
core_clk is enabled.
Originally, that would result in a try to disable the core clock, and if
it was not already enabled, it would result in a kernel warning.
This issue is similar to the one with div_clk.
Fix it by introducing another error path used only between successfully
enabling the core clock, and successfully enabling the div clock.
That would ensure that core clock is disabled only if it was enabled before.

Signed-off-by: Michał Zegan <webczat@webczatnet.pl>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>