[pid_controller] Fix logic for feedforward_mode with single reference interface (backport #1520)

[mergify]

Address #1270. The scope of the fix only covers single reference interface. The changes for two reference interfaces will be covered in different PR.

Testing of the code changes was done with ros-controls/ros2_control_demos#710.

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This is an automatic backport of pull request #1520 done by Mergify.

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 28 lines in your changes missing coverage. Please review.

Project coverage is 66.82%. Comparing base (92c47bd) to head (5984ba7).
Report is 1 commits behind head on humble.

Files with missing lines	Patch %	Lines
pid_controller/test/test_pid_controller.cpp	61.90%	0 Missing and 24 partials ⚠️
pid_controller/src/pid_controller.cpp	42.85%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           humble    #1539      +/-   ##
==========================================
- Coverage   66.84%   66.82%   -0.02%     
==========================================
  Files         114      114              
  Lines       13459    13525      +66     
  Branches     8334     8386      +52     
==========================================
+ Hits         8996     9038      +42     
+ Misses       1436     1433       -3     
- Partials     3027     3054      +27     

Flag	Coverage Δ
unittests	66.82% <60.00%> (-0.02%)	⬇️

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

Files with missing lines	Coverage Δ
pid_controller/test/test_pid_controller.hpp	80.37% <ø> (+0.93%)	⬆️
pid_controller/src/pid_controller.cpp	66.22% <42.85%> (-1.35%)	⬇️
pid_controller/test/test_pid_controller.cpp	50.80% <61.90%> (+2.24%)	⬆️

... and 3 files with indirect coverage changes