ASoC: rt5682-i2c: Use devm_clk_get_optional for optional clock

The mclk clock is optional, but it's currently using devm_clk_get:
simplify the handling by using devm_clk_get_optional instead.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20211026081030.422481-1-angelogioacchino.delregno@collabora.com


Signed-off-by: Mark Brown <broonie@kernel.org>