AP_Notify: Display_SSD1306_I2C: Fix after conversion to I2CDevice

- Initialize device on hw_init() method, allowing it not to be
      present
    - Add missing lock
    - Add packed attribute to structs
    - Move defines to source file

libraries/AP_Notify/Display_SSD1306_I2C.cpp

@@ -16,14 +16,16 @@
  */
 #include "Display_SSD1306_I2C.h"
 
+#include <utility>
+
 #include <AP_HAL/AP_HAL.h>
+#include <AP_HAL/I2CDevice.h>
+
+#define SSD1306_I2C_BUS 2
+#define SSD1306_I2C_ADDR 0x3C    // default I2C bus address
 
 static const AP_HAL::HAL& hal = AP_HAL::get_HAL();
 
-Display_SSD1306_I2C::Display_SSD1306_I2C():
-    _dev(hal.i2c_mgr->get_device(2, SSD1306_I2C_ADDRESS))
-{
-}
 
 bool Display_SSD1306_I2C::hw_init()
 {
@@ -36,8 +38,10 @@ bool Display_SSD1306_I2C::hw_init()
                           0xD9, 0xF1, 0xDB, 0x40, 0xA4, 0xA6,
                           0xAF, 0x21, 0, 127, 0x22, 0, 7 } };
 
+    _dev = std::move(hal.i2c_mgr->get_device(SSD1306_I2C_BUS, SSD1306_I2C_ADDR));
+
     // take i2c bus sempahore
-    if (!_dev->get_semaphore()->take(HAL_SEMAPHORE_BLOCK_FOREVER)) {
+    if (!_dev || !_dev->get_semaphore()->take(HAL_SEMAPHORE_BLOCK_FOREVER)) {
         return false;
     }
 
@@ -55,15 +59,19 @@ bool Display_SSD1306_I2C::hw_init()
 
 bool Display_SSD1306_I2C::hw_update()
 {
-    struct {
+    struct PACKED {
         uint8_t reg;
         uint8_t cmd[6];
     } command = { 0x0, {0x21, 0, 127, 0x22, 0, 7} };
 
-    struct {
+    struct PACKED {
         uint8_t reg;
         uint8_t db[SSD1306_ROWS];
-    } display_buffer = { 0x40 };
+    } display_buffer = { 0x40, {} };
+
+    if (!_dev || !_dev->get_semaphore()->take(5)) {
+        return false;
+    }
 
     // write buffer to display
     for (uint8_t i = 0; i < (SSD1306_COLUMNS / SSD1306_COLUMNS_PER_PAGE); i++) {

libraries/AP_Notify/Display_SSD1306_I2C.h

@@ -4,14 +4,12 @@
 #include "Display.h"
 #include <AP_HAL/I2CDevice.h>
 
-#define SSD1306_I2C_ADDRESS 0x3C    // default I2C bus address
 #define SSD1306_ROWS 128		    // display rows
 #define SSD1306_COLUMNS 64		    // display columns
 #define SSD1306_COLUMNS_PER_PAGE 8
 
 class Display_SSD1306_I2C: public Display {
 public:
-    Display_SSD1306_I2C();
     virtual bool hw_init();
     virtual bool hw_update();
     virtual bool set_pixel(uint16_t x, uint16_t y);