AP_HAL_Linux: I2CDevice: move implementation to outside

As we add methods, it's becoming too complex to be implemented together
with the declaration.

libraries/AP_HAL_Linux/I2CDevice.cpp

@@ -75,15 +75,24 @@ static inline char *startswith(const char *s, const char *prefix)
 /* Private struct to maintain for each bus */
 class I2CBus {
 public:
-    ~I2CBus()
-    {
+    ~I2CBus();
+    int open(uint8_t n);
+
+    Semaphore sem;
+    int fd = -1;
+    uint8_t bus;
+    uint8_t ref;
+};
+
+I2CBus::~I2CBus()
+{
     if (fd >= 0) {
         ::close(fd);
     }
-    }
+}
 
-    int open(uint8_t n)
-    {
+int I2CBus::open(uint8_t n)
+{
     char path[sizeof("/dev/i2c-XXX")];
     int r;
 
@@ -104,14 +113,7 @@ public:
     bus = n;
 
     return fd;
-    }
-
-    Semaphore sem;
-    int fd = -1;
-    uint8_t bus;
-
-    uint8_t ref;
-};
+}
 
 I2CDevice::~I2CDevice()
 {