Rover: move STR_STR_RAT_FF above _P

Some Rover users are having trouble following the [wiki advice on tuning steering](http://ardupilot.org/rover/docs/rover-tuning-steering-rate.html). In particular users seem to often start tuning P and I first instead of the far more important feed-forward parameter. I hope/expect that if we moved FF to the top of the screen they might start with this parameter first.


Start at the Rover basic tuning screen and locate the STR_STR_RAT_FF and _P controls shown in the issue screenshot. Move STR_STR_RAT_FF above _P, then verify that the Rover tuning UI presents feed-forward before P and that the screen still behaves correctly.

Written by the indexing model from the issue text.