ASoC: rt5645: fix error handling for gpio detection

Optional gpio handling should not cause an error status and prevent
probing if it's missing.  Remove error return for -ENOENT case and
move error message to dev_info

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>