drm/amdgpu: fix no bad_pages issue after umc ue injection

old code wrongly used the bad page status as the function return value,
which cause amdgpu_ras_badpages_read always return failed.

Signed-off-by: Dennis Li <Dennis.Li@amd.com>
Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>