steering_controllers: Fix deprecation warning

[christophfroehlich]

I made a mistake with #1093: The deprecation warning was inserted in the wrong place.
Will be removed with #1151 on rolling, but let's fix this on iron.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 86.79%. Comparing base (ced16a8) to head (1326505).

Additional details and impacted files

@@            Coverage Diff             @@
##             iron    #1155      +/-   ##
==========================================
+ Coverage   86.69%   86.79%   +0.09%     
==========================================
  Files          86       86              
  Lines        7411     7413       +2     
  Branches      612      613       +1     
==========================================
+ Hits         6425     6434       +9     
+ Misses        758      750       -8     
- Partials      228      229       +1     

Flag	Coverage Δ
unittests	86.79% <0.00%> (+0.09%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...llers_library/src/steering_controllers_library.cpp	74.16% <0.00%> (-0.32%)	⬇️

... and 3 files with indirect coverage changes