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: NWei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: NArnd Bergmann <arnd@arndb.de>
Acked-by: NLee Jones <lee.jones@linaro.org>
Signed-off-by: NMark Brown <broonie@linaro.org>