AP_RCProtocol: Add missing const in member functions

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

libraries/AP_RCProtocol/AP_RCProtocol.h

@@ -45,7 +45,7 @@ public:
         NONE    //last enum always is None
     };
     void init();
-    bool valid_serial_prot()
+    bool valid_serial_prot() const
     {
         return _valid_serial_prot;
     }

libraries/AP_RCProtocol/AP_RCProtocol_Backend.cpp

@@ -37,7 +37,7 @@ bool AP_RCProtocol_Backend::new_input()
     return ret;
 }
 
-uint8_t AP_RCProtocol_Backend::num_channels()
+uint8_t AP_RCProtocol_Backend::num_channels() const
 {
     return _num_channels;
 }

libraries/AP_RCProtocol/AP_RCProtocol_Backend.h

@@ -32,7 +32,7 @@ public:
     uint16_t read(uint8_t chan);
     void read(uint16_t *pwm, uint8_t n);
     bool new_input();
-    uint8_t num_channels();
+    uint8_t num_channels() const;
 
     // support for receivers that have FC initiated bind support
     virtual void start_bind(void) {}