ASoC: ux500: Fix sparse non static symbol warning

Fixes the following sparse warning:

sound/soc/ux500/ux500_msp_i2s.c:649:5: warning:
 symbol 'ux500_msp_i2s_of_init_msp' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>