drm/amdgpu/swsmu: make sienna cichlid function static

Unused outside of the file.
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/pm/swsmu/smu11/sienna_cichlid_ppt.c

@@ -3832,7 +3832,7 @@ static void sienna_cichlid_stb_init(struct smu_context *smu)
 
 }
 
-int sienna_cichlid_stb_get_data_direct(struct smu_context *smu,
+static int sienna_cichlid_stb_get_data_direct(struct smu_context *smu,
 					      void *buf,
 					      uint32_t size)
 {