drm/amdgpu/debugfs: fix error code when smc register accessors are NULL

Should be -EOPNOTSUPP.

Fixes: 5104fdf5 ("drm/amdgpu: Fix a null pointer access when the smc_rreg pointer is NULL")
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>