ASoC: ux500: forward declare msp_i2s_platform_data

We get a lot of build warnings from the msp driver like:

In file included from sound/soc/ux500/ux500_msp_dai.h:21:0,
                 from sound/soc/ux500/mop500.c:25:
sound/soc/ux500/ux500_msp_i2s.h:546:11: warning: 'struct msp_i2s_platform_data' declared inside parameter list [enabled by default]
    struct msp_i2s_platform_data *platform_data);
           ^
sound/soc/ux500/ux500_msp_i2s.h:546:11: warning: its scope is only this definition or declaration, which is probably not what you want [enabled by default]

The easiest solution is to add a declaration of the struct name.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>