fix: correctly convert RegisteredPoStProof type

[arajasek]

#1748, but on the release branch

[codecov-commenter]

Codecov Report

Merging #1750 (f8c20ab) into release/v3.2 (458262d) will decrease coverage by 19.21%.
The diff coverage is 0.00%.

Additional details and impacted files

@@                Coverage Diff                @@
##           release/v3.2    #1750       +/-   ##
=================================================
- Coverage         74.76%   55.55%   -19.21%     
=================================================
  Files               146      146               
  Lines             14093    14021       -72     
=================================================
- Hits              10536     7790     -2746     
- Misses             3557     6231     +2674     

Impacted Files	Coverage Δ
shared/src/sector/registered_proof.rs	0.82% <0.00%> (-5.77%)	⬇️

... and 58 files with indirect coverage changes

[raulk]

I'm not familiar with the actual new proof enum types on the fil proofs library, but this fix looks correct given that we were returning the old values before.