mmc: mtk-sd: Assign src_clk parent to src_clk_cg for legacy DTs

mmc: mtk-sd: Assign src_clk parent to src_clk_cg for legacy DTs

In commit 3c1a88443698 ("mmc: mediatek: add support of source_cg clock")
an independent cg was introduced to avoid a hardware hang issue during
clock mode switches (subsequent commits will set that clock as optional).

When this clock is not present in device-tree, any operation is being
done on src_clk's parent (calling clk_get_parent()): to simplify this
and avoid checking for src_clk_cg presence everytime, just assign the
parent clock to src_clk_cg and remove the now useless checks.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20211216125748.179602-5-angelogioacchino.delregno@collabora.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>