transport: refactor trInFlow.onData to eliminate redundant logic

[luxcgo]

RELEASE NOTES: None

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.17%. Comparing base (b850ea5) to head (b2a3a62).
Report is 10 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7734      +/-   ##
==========================================
+ Coverage   81.96%   82.17%   +0.21%     
==========================================
  Files         362      362              
  Lines       28119    28116       -3     
==========================================
+ Hits        23048    23105      +57     
+ Misses       3863     3826      -37     
+ Partials     1208     1185      -23     

Files with missing lines	Coverage Δ
internal/transport/flowcontrol.go	96.77% <100.00%> (-0.11%)	⬇️

... and 22 files with indirect coverage changes

[arjan-bal]

LGTM, adding a second reviewer.