drm/amd/powerplay/hwmgr: prevent VDDC from exceeding 2V

If the tonga gpu is controlled by SVID2 tonga_get_evv_voltage will only print
an error if the voltage exceeds 2V although a comment clearly states that it
needs be less than 2V.

v2: added signed of by

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