rtl8720dn: add handling when connect fails

[sago35]

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This PR adds handling when rtl8720dn fails to connect to a specific TCP destination.

-1 represents many rpc failures, but details are unknown. In fact, rtl8720dn returns -1 when a connection fails.

[scottfeldman]

Note in drivers:dev branch, this is fixed as:

result := r.rpc_lwip_connect(int32(sock), name, uint32(len(name)))
if result == -1 {
        return fmt.Errorf("Connect to %s failed", ip)
}

[deadprogram]

@sago35 is this PR still relevant?

[sago35]

No, it's already been updated to the following code, so the issue is resolved. I'll close this PR.

https://github.com/tinygo-org/drivers/blob/v0.27.0/rtl8720dn/rtl8720dn.go#L501-L505