mmc: mtk-sd: Fix usage of devm_clk_get_optional()

mmc: mtk-sd: Fix usage of devm_clk_get_optional()

If we get an error during probe of an optional clock with function
devm_clk_get_optional(), this means that the clock was provided, but
an error occurred: this has to be escalated to the probe function
for the driver probe to fail accordingly, or unexpected hardware
behavior may happen.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20211216125748.179602-4-angelogioacchino.delregno@collabora.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>