ASoC: ux500: Drop dangling struct i2s_controller

Drop struct i2s_controller from the ux500 ASoC driver as right now it is
instantiated but not used anywhere.  Also drop a mismatched
device_unregister in the process.

Signed-off-by: Fabio Baltieri <fabio.baltieri@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>