ASoC: rt5640: fix sparse warnings

This fixes:
975:9: sparse: Using plain integer as NULL pointer
1917:24: sparse: symbol 'rt5640_aif_dai_ops' was not declared. Should it be static?
1924:27: sparse: symbol 'rt5640_dai' was not declared. Should it be static?
2079:19: sparse: symbol 'rt5640_i2c_driver' was not declared. Should it be static?

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@linaro.org>