net: stmmac: don't put an absent MDIO reset GPIO

[sormy]

devm_gpiod_get_optional() returns NULL when the property is absent, but both call
sites only test IS_ERR() before devm_gpiod_put(). Putting NULL means
devres_release() finds nothing, returns -ENOENT, and the WARN_ON inside
devm_gpiod_put() fires on every probe:

WARNING: CPU: 1 PID: 1 at drivers/gpio/gpiolib-devres.c:327 devm_gpiod_put+0x34/0x44
 devm_gpiod_put → stmmac_mdio_reset → __mdiobus_register

Only three device trees in this repo set snps,reset, so most Rockchip boards with
an integrated PHY take the NULL path. gpiod_set_value_cansleep() already tolerates
NULL, so only the two puts need guarding.

Verified on an RK3528 board: MDIO registers (stmmac-0:02), end0 comes up, warning
gone.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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Configuration used: Organization UI

Review profile: CHILL

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Run ID: b0675efd-759b-4494-accf-ca387b9c0a97

📥 Commits

Reviewing files that changed from the base of the PR and between 5280f9b and a44fa53.

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• drivers/net/ethernet/stmicro/stmmac/stmmac_mdio.c

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Walkthrough

The STMMAC MDIO driver now checks whether the optional reset GPIO descriptor exists before releasing it in stmmac_mdio_idle and stmmac_mdio_reset. This removes cleanup calls for null descriptors.

Estimated code review effort: 1 (Trivial) | ~2 minutes

Merge Risk: ⚪ Minimal · up to a44fa

This localized change safely avoids releasing an absent MDIO reset GPIO descriptor; no actionable merge-blocking risk remains after normal checks and review.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely describes the fix for absent MDIO reset GPIO handling.
Description check	✅ Passed	The description explains the NULL GPIO issue, the warning, the code fix, and verification results.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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