fix: exhaust egress socket state on poll

[thomaseizinger]

At present, smoltcp only does a single pass of advancing the egress socket state. This means the layer above has to call poll in a loop to ensure sufficient progress has been made.

We already exhaustively progress the ingress state further up so doing the same thing for the egress state seems appropriate.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 81.17%. Comparing base (e2b75e3) to head (e528210).

Files with missing lines	Patch %	Lines
src/iface/interface/mod.rs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1059   +/-   ##
=======================================
  Coverage   81.17%   81.17%           
=======================================
  Files          81       81           
  Lines       28955    28955           
=======================================
  Hits        23503    23503           
  Misses       5452     5452           

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