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

if atomctrl_get_voltage_evv_on_sclk returns non zero (fail) in the expansion
of the PP_ASSERT_WITH_CODE macro the continue will actually do nothing
(The macro uses a do ... while(0) as scope, which eats the continue).
Based on the code I don't think this was the intent.
Unfortunately fixing this requires rewriting the control flow and
removing the macros.

v2: added signed of by
fixed error message print

v3: agd: drop DRM_ERROR

Signed-off-by: Moritz Kühner <kuehner.moritz@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>