vendor._siliconblue: fix off-by-1 with default_clk = "SB_LFOSC"

[whitequark]

This bug caused all logic in the sync domain to be optimized out, as the reset ended up being continuously asserted.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.28%. Comparing base (3a3bf81) to head (40ba283).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1650   +/-   ##
=======================================
  Coverage   91.28%   91.28%           
=======================================
  Files          44       44           
  Lines       11478    11478           
  Branches     2237     2237           
=======================================
  Hits        10478    10478           
  Misses        837      837           
  Partials      163      163           

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