drm/amdgpu: correct ras error count type

Use unsigned long type for the same ras count variable.
This will avoid overflow on 64 bit system.

Signed-off-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Tao Zhou <tao.zhou1@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>