ASoC: Intel: fix sparse non static symbol warnings

Fixes the following sparse warnings:

sound/soc/intel/sst/sst_acpi.c:248:5: warning:
 symbol 'sst_acpi_probe' was not declared. Should it be static?
sound/soc/intel/sst/sst_acpi.c:335:5: warning:
 symbol 'sst_acpi_remove' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Mark Brown <broonie@kernel.org>