ASoC: Intel: bytcht_es8316: Switch to use gpiod_get_optional()

First of all, replace indexed API by plain one since we have index 0.
Second, switch to optional variant and drop duplicated code.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Message-Id: <20211007164523.27094-4-andriy.shevchenko@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>