clarifying i2c_get_chip_for_busnum()

[Sahaj Sarup]

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!