drm/amdgpu: remove is_mca_add for ras_add_bad_pages

Remove unnecessary variable and simplify the logic.

Signed-off-by: Tao Zhou <tao.zhou1@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>