lwip_sock: Make sock_tcp_read return data if available
lwip_sock: Make sock_tcp_read return data if available

When reading from the socket with `sock_tcp_read()` it would only return
data from at most one internal connection buffer, even if the buffer
passed to `sock_tcp_read()` is larger and there is more data available
in the connection. This patch makes `sock_tcp_read` process all the
available data so long as there's more data to read available
immediately.

kaspar030/RIOT,jasonatran/RIOT,ant9000/RIOT,RIOT-OS/RIOT,ant9000/RIOT,kaspar030/RIOT,OTAkeys/RIOT,jasonatran/RIOT,kYc0o/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,miri64/RIOT,OTAkeys/RIOT,ant9000/RIOT,authmillenon/RIOT,OlegHahm/RIOT,kaspar030/RIOT,kYc0o/RIOT,ant9000/RIOT,ant9000/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,jasonatran/RIOT,miri64/RIOT,kYc0o/RIOT,authmillenon/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,miri64/RIOT,authmillenon/RIOT,kYc0o/RIOT,authmillenon/RIOT,authmillenon/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,kaspar030/RIOT,OTAkeys/RIOT,miri64/RIOT,kaspar030/RIOT,OTAkeys/RIOT,miri64/RIOT,OlegHahm/RIOT