RC_Channel: support up to 12 aux channels on PX4

this allows RC13 and RC14 to work correctly as aux outputs on Pixhawk

libraries/RC_Channel/RC_Channel_aux.cpp

@@ -19,7 +19,7 @@ const AP_Param::GroupInfo RC_Channel_aux::var_info[] PROGMEM = {
     AP_GROUPEND
 };
 
-RC_Channel_aux *RC_Channel_aux::_aux_channels[8];
+RC_Channel_aux *RC_Channel_aux::_aux_channels[RC_AUX_MAX_CHANNELS];
 uint32_t RC_Channel_aux::_function_mask;
 
 /// map a function to a servo channel and output it

libraries/RC_Channel/RC_Channel_aux.h

@@ -7,9 +7,14 @@
 #ifndef __RC_CHANNEL_AUX_H__
 #define __RC_CHANNEL_AUX_H__
 
+#include <AP_HAL.h>
 #include "RC_Channel.h"
 
+#if HAL_CPU_CLASS > HAL_CPU_CLASS_16
+#define RC_AUX_MAX_CHANNELS 12
+#else
 #define RC_AUX_MAX_CHANNELS 8
+#endif
 
 /// @class	RC_Channel_aux
 /// @brief	Object managing one aux. RC channel (CH5-8), with information about its function