drm/amdgpu/vcn: Return void from the stop_dbg_mode

There is no point in returning an int here. It only returns 0 which
the caller never uses. Therefore return void and remove the unnecessary
assignment.

Addresses-Coverity: 1504988 ("Unused value")
Fixes: 8da1170a ("drm/amdgpu: add VCN4 ip block support")
Reviewed-by: Ruijing Dong <ruijing.dong@amd.com>
Suggested-by: Ruijing Dong <ruijing.dong@amd.com>
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Khalid Masum <khalid.masum.92@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>