Fix following build error when CONFIG_I2C is not enabled:

sound/soc/codecs/rt5640.c:2252:1: warning: data definition has no type or storage class
 module_i2c_driver(rt5640_i2c_driver);
 ^
sound/soc/codecs/rt5640.c:2252:1: error: type defaults to ‘int’ in declaration of ‘module_i2c_driver’ [-Werror=implicit-int]
sound/soc/codecs/rt5640.c:2252:1: warning: parameter names (without types) in function declaration
sound/soc/codecs/rt5640.c:2241:26: warning: ‘rt5640_i2c_driver’ defined but not used [-Wunused-variable]
 static struct i2c_driver rt5640_i2c_driver = {
                          ^
cc1: some warnings being treated as errors
Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Mark Brown <broonie@kernel.org>