Handle callbacks dynamically.

See merge request !42

coupler/mod_io_i2c.h

@@ -12,7 +12,7 @@ const int DEFAULT_I2C_SLAVE_ADDR = 0x58;
 int I2C_SLAVE_ADDR_LIST[127] = {0};
 
 // the number of physical relays
-int DEFAULT_RELAY_COUNT = 4;
+const int DEFAULT_RELAY_COUNT = 4;
 
 // the block device at host machine
 static char *DEFAULT_I2C_BLOCK_DEVICE_NAME = "/dev/i2c-1";