drm/amd/pm: Fix incorrect power limit readback in smu11 if POWER_SOURCE_DC

 when smu->adev->pm.ac_power == 0, message parameter with bit 16 set is saved
 to smu->current_power_limit.

Fixes: 0cb4c621 ("drm/amd/pm: correct power limit setting for SMU V11)"

Signed-off-by: Darren Powell <darren.powell@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>