Change method name 'little' to 'getLittleField'

[pdhung3012]

This class is used to represent Field. This method named 'little' is to get a LittleField object. Thus, the method name 'getLittleField' is more intuitive than 'little'.

[5iver]

#6883

https://github.com/openhab/openhab2-addons/tree/master/bundles/org.openhab.binding.lifx