Do not reset TCP connections when using a RawSocket

When the socket set only contains a raw socket, all TCP connections will be reset by the code here: https://github.com/smoltcp-rs/smoltcp/blob/e000f15ebc8b695c481ad8e12bef1536077a422f/src/iface/interface/tcp.rs#L40-L41 However, when using a raw socket which can accept TCP, this is unlikely to be desired behaviour. This code should probably be modified to check if any raw sockets are able to accept the packet before deciding whether to send an RST. I'm happy to have a go at fixing this, let me know if this sounds unreasonable.