ASoC: intel - atom: statify sst_slot_enum_info

Sparse complains that sst_slot_enum_info should be static, so make it
static

sound/soc/intel/atom/sst-atom-controls.c:135:5: warning: symbol
'sst_slot_enum_info' was not declared. Should it be static?

Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Acked-by: Liam Girdwood <liam.r.girdwood@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>