AP_Battery: fixed bug in SUI driver

fixed bug in total voltage

libraries/AP_BattMonitor/AP_BattMonitor_SMBus_SUI.cpp

@@ -139,10 +139,11 @@ void AP_BattMonitor_SMBus_SUI::read_cell_voltages()
             pack_voltage_mv = total_mv;
         } else {
             // we can't get total pack voltage. Use average of cells we have so far
+            const uint16_t cell_mv = pack_voltage_mv / SUI_MAX_CELL_READ;
             for (uint8_t i = SUI_MAX_CELL_READ; i < cell_count; i++) {
-                _state.cell_voltages.cells[i] = pack_voltage_mv / SUI_MAX_CELL_READ;
-                pack_voltage_mv += _state.cell_voltages.cells[i];
+                _state.cell_voltages.cells[i] = cell_mv;
             }
+            pack_voltage_mv += cell_mv * (cell_count - SUI_MAX_CELL_READ);
         }
     }