HAL_Linux: AnalogIn_IIO, replace reopen by select

A new select_pin method has been implemented and
set that changes solely the file descriptor (avoids
reopening the file every time we call set_pin).

libraries/AP_HAL_Linux/AnalogIn_IIO.cpp

@@ -30,7 +30,8 @@ IIOAnalogSource::IIOAnalogSource(int16_t pin, float initial_value) :
     _sum_count(0),
     _pin_fd(-1)
 {
-    reopen_pin();
+    init_pins();
+    select_pin();    
 }
 
 void IIOAnalogSource::init_pins(void)
@@ -48,6 +49,17 @@ void IIOAnalogSource::init_pins(void)
     }
 }
 
+/*
+  selects a diferent file descriptor among in the fd_analog_sources array
+ */
+void IIOAnalogSource::select_pin(void)
+{
+    _pin_fd = fd_analog_sources[_pin];
+}
+
+/*
+  reopens an analog source (by closing and opening it again)
+ */
 void IIOAnalogSource::reopen_pin(void)
 {
     char buf[100];
@@ -139,7 +151,7 @@ void IIOAnalogSource::set_pin(uint8_t pin)
     _sum_count = 0;
     _latest = 0;
     _value = 0;    
-    reopen_pin();
+    select_pin();
     // _value_ratiometric = 0;
     hal.scheduler->resume_timer_procs();
 }

libraries/AP_HAL_Linux/AnalogIn_IIO.h

@@ -42,12 +42,13 @@ private:
     uint8_t     _sum_count;
     int16_t     _pin;
     int         _pin_fd;    
+    int         fd_analog_sources[IIO_ANALOG_IN_COUNT];
 
     void reopen_pin(void);
     void init_pins(void);
+    void select_pin(void);
 
     static const char *analog_sources[IIO_ANALOG_IN_COUNT];
-    static int fd_analog_sources[IIO_ANALOG_IN_COUNT];
 };
 
 class IIOAnalogIn : public AP_HAL::AnalogIn {