ASoC: rt5659: declare rt5659_i2c_driver static

Declar rt5659_i2c_driver, which is only being passed to
module_i2c_driver(rt5659_i2c_driver), static.

Fixes: commit d3cb2de2 ("ASoC: rt5659: add rt5659 codec driver")
Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: Mark Brown <broonie@kernel.org>