drm/amdgpu/pm: fix powerplay OD interface

The overclocking interface currently appends data to a
string.  Revert back to using sprintf().

Bug: https://gitlab.freedesktop.org/drm/amd/-/issues/1774


Fixes: 6db0c87a ("amdgpu/pm: Replace hwmgr smu usage of sprintf with sysfs_emit")
Acked-by: Evan Quan <evan.quan@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>