extmod/network_cyw43: Fix STA default netif after AP mode switch.

[prafulfillment]

Summary

After switching from AP mode back to STA mode, netif_default remains pointing to the AP netif. This causes all outbound TCP connections to fail with EHOSTUNREACH (errno 113) because traffic routes through the now-defunct AP gateway instead of the STA gateway.

The root cause is in cyw43_wifi_set_up() in cyw43_ctrl.c: when STA is reactivated, itf_state bit 0 is still set from the initial boot, so cyw43_tcpip_init(STA) is skipped entirely and with it, the only call to netif_set_default(STA).

This fix adds an explicit netif_set_default() call in network_cyw43_active() whenever STA is activated, ensuring STA is always the default route regardless of itf_state.

Testing

Tested on OpenMV AE3

• Boot in STA mode, verify relay server reachable — PASS
• Switch to AP mode via BLE, verify AP clients can connect — PASS
• Switch back to STA mode, verify relay server reachable without deinit() or machine.reset() — PASS
• Rapid AP↔STA cycling 10x — PASS, no errno 113

Trade-offs and Alternatives

The fix is a redundant netif_set_default() call on every STA activation, even when STA is already the default.

The ideal fix would be in cyw43_wifi_set_up() in cyw43_ctrl.c (the upstream cyw43-driver), either by clearing itf_state bit 0 on STA teardown or by calling cyw43_tcpip_deinit(STA) in the up=false path. However, that driver is maintained separately. This fix in the MicroPython binding layer is self-contained and does not require upstream driver changes.

The alternative that I tried and worked is calling wlan.deinit() which resets the entire CYW43 chip, killing BLE and requiring full reinitialization (~3-4 seconds of downtime).

[iabdalkader]

Hi! Can you send this upstream please? Once it's merged, I'll cherry-pick it back here.