Portenta H7: fix multi-client connection issues

[leonardocavagnis]

This PR updates the default Zephyr configuration for the Arduino Portenta H7 to properly support multiple simultaneous TCP connections.

The previous configuration used conservative defaults that limited the network stack's ability to handle more than one active socket alongside the listening server.

This PR resolves the issues arising from #227, where only a single client could connect at a time when using the AdvancedChatServer.ino example, preventing Connection refused errors for subsequent clients.

[github-actions]

Built 0.53.2-0.dev+3af2311

CI run PASSED 🟢

Artifact	Board	Core	Tests	RAM	Sketches	Warnings	Errors
✅* zephyr_contrib	ek_ra8d1	📗	✅*	11.9%	2	2	-
	frdm_mcxn947	3 🏷️	✅*	58.0%	2	2	-
	frdm_rw612	1 🏷️	✅*	83.0%	2	2	-
✔️* zephyr_main	giga	4 🏷️	✅*	54.5%	44	14	-
	nano33ble	1 🏷️	✅*	78.7%	22	10	-
	nano_matter	📗	✔️*	⚠️ 85.7%	20	10	(2*)
	niclasense	2 🏷️	✅*	⚠️ 87.3%	20	10	-
	opta	4 🏷️	✔️*	46.7%	54	26	(2*)
	portentac33	3 🏷️	✔️*	⚠️ 95.1%	56	28	(8*)
	portentah7	3 🏷️	✔️*	48.0%	58	28	(2*)
✅* zephyr_unoq	unoq	📗	✅*	26.3%	52	10	-

Legend

Board	Test	Status description
🔥	🔥	Test run failed to complete.
❌	🔴	Test completed with unexpected errors.
✔️*	🚫	Test completed with errors, but all are known/expected.
✅*	🟡	Test completed with some warnings; no errors detected.
✅	🟢	Test passed successfully, with no warnings or errors.
🌑	🌑	Test was skipped.

[CLAassistant]

All committers have signed the CLA.