drm/amd/powerplay: propagate errors in phm_get_voltage_evv_on_sclk

Missing for one case.

bugs:
https://bugzilla.kernel.org/show_bug.cgi?id=185681
https://bugs.freedesktop.org/show_bug.cgi?id=98357



Reviewed-by: Rex Zhu <Rex.Zhu@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>