drm/amd/powerplay: correct data type to support under voltage

For under voltage, negative value will be applied to voltage
offset. Update the data type to cover this case.
Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/powerplay/inc/smu11_driver_if.h

@@ -569,11 +569,11 @@ typedef struct {
   uint16_t      GfxclkFmin;
   uint16_t      GfxclkFmax;
   uint16_t      GfxclkFreq1;
-  uint16_t      GfxclkOffsetVolt1;
+  int16_t      GfxclkOffsetVolt1;
   uint16_t      GfxclkFreq2;
-  uint16_t      GfxclkOffsetVolt2;
+  int16_t      GfxclkOffsetVolt2;
   uint16_t      GfxclkFreq3;
-  uint16_t      GfxclkOffsetVolt3;
+  int16_t      GfxclkOffsetVolt3;
   uint16_t      UclkFmax;
   int16_t       OverDrivePct;
   uint16_t      FanMaximumRpm;