drivers/periph/i2c: Fixed i2c_release() signature

i2c_release() should not have a return value, as:

- There is no reasonable error handling possible by the caller, so there is no
  value in indicating success/failure via the return value to the caller
- There is no legitimate reason to fail *unless* an invalid I2C bus was released
  or an I2C bus that was not previously acquired was released
    --> This would indicate a bug in the code and should be tackled by an
        assert()

drivers/include/periph/i2c.h

@@ -231,10 +231,8 @@ int i2c_acquire(i2c_t dev);
  * @brief   Release the given I2C device to be used by others
  *
  * @param[in] dev           I2C device to release
- *
- * @return                  0 on success, -1 on error
  */
-int i2c_release(i2c_t dev);
+void i2c_release(i2c_t dev);
 
 /**
  * @brief   Convenience function for reading one byte from a given register