ASoC: Intel: bytcr_rt5640: use devm_clk_get_optional() for mclk

ASoC: Intel: bytcr_rt5640: use devm_clk_get_optional() for mclk

The devm_clk_get_optional() helper returns NULL when devm_clk_get()
returns -ENOENT. This makes things slightly cleaner. The added benefit
is mostly cosmetic.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Tested-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20211007165715.27463-4-andriy.shevchenko@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>