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

[ Upstream commit afe58346d5d3887b3e49ff623d2f2e471f232a8d ]

Should be -EOPNOTSUPP.

Fixes: 5104fdf50d32 ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>