ASoC: ux500: Drop unused code from msp headers

Drop unused fields and structures from ux500_msp_i2s header file, as
those looks like leftover from a previous implementation of the driver.

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