drm/amd/powerplay: don't add invalid voltage.

if atomctrl_get_voltage_evv_on_sclk_ai returns
non zero (fail) in the expansion of the
PP_ASSERT_WITH_CODE macro the continue will
actually do nothing, So invalid voltage will be
added to ppbable.

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