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

mainline inclusion
from mainline-v6.8-rc1
commit afe58346d5d3887b3e49ff623d2f2e471f232a8d
category: bugfix
bugzilla: https://gitee.com/src-openeuler/kernel/issues/I9RE4X
CVE: CVE-2023-52817

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=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: Ye Bin <yebin10@huawei.com>