rbac: fix usage of AuthInfo

[dfawley]

This could actually lead to a panic if the auth type reports "tls" but the AuthInfo is not a credentials.TLSInfo. There is no evidence this is happening and has been this way for a long time.

RELEASE NOTES: none

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.90%. Comparing base (b45fc41) to head (b13934e).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7522      +/-   ##
==========================================
- Coverage   82.07%   81.90%   -0.18%     
==========================================
  Files         360      360              
  Lines       27533    27531       -2     
==========================================
- Hits        22599    22550      -49     
- Misses       3759     3788      +29     
- Partials     1175     1193      +18     

Files	Coverage Δ
internal/xds/rbac/rbac_engine.go	82.81% <100.00%> (-0.27%)	⬇️

... and 16 files with indirect coverage changes

[arvindbr8]

you have my authoritative LGTM