hal_ap_linux: Make PRU constants private to reduce #define namespace pollution.

Forthcoming Zynq port AP_HAL_Linux uses similar constants with different
values - move these as private member constants to avoid collisions.

Signed-off-by: John Williams <john@whelanwilliams.net>

libraries/AP_HAL_Linux/RCInput_PRU.h

@@ -12,7 +12,6 @@
 #define RCIN_PRUSS_SHAREDRAM_BASE   0x4a312000
 // we use 300 ring buffer entries to guarantee that a full 25 byte
 // frame of 12 bits per byte
-#define NUM_RING_ENTRIES            300
 
 class Linux::LinuxRCInput_PRU : public Linux::LinuxRCInput 
 {
@@ -21,6 +20,7 @@ public:
     void _timer_tick(void);
 
  private:
+    static const unsigned int NUM_RING_ENTRIES=300;
     // shared ring buffer with the PRU which records pin transitions
     struct ring_buffer {
         volatile uint16_t ring_head; // owned by ARM CPU

libraries/AP_HAL_Linux/RCOutput_PRU.h

@@ -7,8 +7,6 @@
 #define MAX_PWMS                 12
 #define PWM_CMD_MAGIC            0xf00fbaaf
 #define PWM_REPLY_MAGIC          0xbaaff00f
-#define TICK_PER_US              200
-#define TICK_PER_S               200000000
 #define PWM_CMD_CONFIG	         0	/* full configuration in one go */
 #define PWM_CMD_ENABLE	         1	/* enable a pwm */
 #define PWM_CMD_DISABLE	         2	/* disable a pwm */
@@ -29,6 +27,8 @@ class Linux::LinuxRCOutput_PRU : public AP_HAL::RCOutput {
     void     read(uint16_t* period_us, uint8_t len);
 
 private:
+    static const int TICK_PER_US=200;
+    static const int TICK_PER_S=200000000;
     struct pwm_cmd {
         uint32_t magic;
         uint32_t enmask;     /* enable mask */