Fix obsolete cnt assignment in _tu_fifo_peek() overflow check

[Copilot]

This PR fixes a logic error identified by static analysis in the _tu_fifo_peek() function where the cnt variable was being assigned but never used.

Issue Analysis:
In the overflow check at lines 427-432 of src/common/tusb_fifo.c, the code was setting cnt = f->depth but this value was never used afterward:

// Check overflow and correct if required
if ( cnt > f->depth )
{
  rd_idx = _ff_correct_read_index(f, wr_idx);
  cnt = f->depth;  // This assignment was never used
}

Comparison with similar functions:

• _tu_fifo_peek_n() uses the corrected cnt value to limit the number of items read: if ( cnt < n ) n = cnt;
• tu_fifo_get_read_info() uses the corrected cnt value to set buffer info: info->len_lin = cnt;
• _tu_fifo_peek() only peeks at a single item and doesn't use cnt after the overflow check

Solution:
Removed the obsolete cnt = f->depth; assignment while preserving the necessary read index correction (rd_idx = _ff_correct_read_index(f, wr_idx);).

Testing:
All existing FIFO tests continue to pass (14/14) and the full unit test suite passes (21/21), confirming the change doesn't affect functionality.

Fixes #3218.

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[Copilot]

@hathach 👋 This repository doesn't have Copilot instructions. With Copilot instructions, I can understand the repository better, work faster and produce higher quality PRs.

I can generate a .github/copilot-instructions.md file for you automatically. Click here to open a pre-filled issue and assign it to me. I'll write the instructions, and then tag you for review.

[Copilot]

Pull Request Overview

This PR fixes a logic error in the _tu_fifo_peek() function where a variable assignment in the overflow check was obsolete and never used, as identified by static analysis.

• Removes the unused cnt = f->depth; assignment from the overflow correction block
• Preserves the necessary read index correction functionality
• Aligns the function behavior with its single-item peek purpose

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[hathach]

good