drm/amdgpu: fix amdgpu_ras_mca_query_error_status scope

This commit fixes the compile-time warning below:

 warning: no previous prototype for ‘amdgpu_ras_mca_query_error_status’
 [-Wmissing-prototypes]

Changes since v1:
- As suggested by Alexander Deucher:
  1. Make function static instead of adding prototype.
Signed-off-by: Isabella Basso <isabbasso@riseup.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_ras.c

@@ -867,9 +867,9 @@ static int amdgpu_ras_enable_all_features(struct amdgpu_device *adev,
 /* feature ctl end */
 
 
-void amdgpu_ras_mca_query_error_status(struct amdgpu_device *adev,
-				       struct ras_common_if *ras_block,
-				       struct ras_err_data  *err_data)
+static void amdgpu_ras_mca_query_error_status(struct amdgpu_device *adev,
+					      struct ras_common_if *ras_block,
+					      struct ras_err_data  *err_data)
 {
 	switch (ras_block->sub_block_index) {
 	case AMDGPU_RAS_MCA_BLOCK__MP0: