ASoC: rt1019: make symbol 'rt1019_i2c_driver' static

ASoC: rt1019: make symbol 'rt1019_i2c_driver' static

The sparse tool complains as follows:

sound/soc/codecs/rt1019.c:927:19: warning:
 symbol 'rt1019_i2c_driver' was not declared. Should it be static?

This symbol is not used outside of rt1019.c, so this
commit marks it static.

Fixes: 7ec79d3850d0 ("ASoC: rt1019: add rt1019 amplifier driver")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Link: https://lore.kernel.org/r/20210319094102.4185096-1-weiyongjun1@huawei.com
Signed-off-by: Mark Brown <broonie@kernel.org>