ASoC: wm8960: Use devm_regmap_init_i2c()

devm_regmap_init_i2c() is device managed and makes error
handling and code cleanup simpler. There was no explicit
regmap_exit call in this function which was probably a bug.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>