clarifying i2c_get_chip_for_busnum()

Hi,

In `include/i2c.h` , the udevice pointer and return value definition seems to be confusing.

``` /** * i2c_get_chip_for_busnum() - get a device to use to access a chip on . . . * @devp: Returns pointer to new device if found or -ENODEV if not * found */ ```

Should this instead be:

``` * @devp: Returns pointer to new device or NULL if not found * Return: 0 on success, -ENODEV on failure ```

Thanks!