ASoC: rt5665: make local symbol rt5665_i2c_driver static

Fixes the following sparse warnings:

sound/soc/codecs/rt5665.c:4928:19: warning:
symbol 'rt5665_i2c_driver' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Mark Brown <broonie@kernel.org>