AP_BattMonitor: Add missing const in member functions

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

libraries/AP_BattMonitor/AP_BattMonitor.h

@@ -185,7 +185,7 @@ public:
     bool overpower_detected(uint8_t instance) const;
 
     // cell voltages in millivolts
-    bool has_cell_voltages() { return has_cell_voltages(AP_BATT_PRIMARY_INSTANCE); }
+    bool has_cell_voltages() const { return has_cell_voltages(AP_BATT_PRIMARY_INSTANCE); }
     bool has_cell_voltages(const uint8_t instance) const;
     const cells &get_cell_voltages() const { return get_cell_voltages(AP_BATT_PRIMARY_INSTANCE); }
     const cells &get_cell_voltages(const uint8_t instance) const;

libraries/AP_BattMonitor/AP_BattMonitor_Params.h

@@ -21,7 +21,7 @@ public:
         Ignore_UAVCAN_SoC = (1U<<0),
     };
 
-    BattMonitor_LowVoltage_Source failsafe_voltage_source(void) { return (enum BattMonitor_LowVoltage_Source)_failsafe_voltage_source.get(); }
+    BattMonitor_LowVoltage_Source failsafe_voltage_source(void) const { return (enum BattMonitor_LowVoltage_Source)_failsafe_voltage_source.get(); }
 
     AP_Int8  _type;                     /// 0=disabled, 3=voltage only, 4=voltage and current
     AP_Int8  _volt_pin;                 /// board pin used to measure battery voltage