Fix DNS get methods

[manchoz]

This PR fixes the ::dnsServerIP() and ::dnsIP() methods to retrieve the DNS server configuration properly.

The issue is due to NetworkInterface.cpp#L111-L112 ultimately requiring the interface name because of lwip_dns.c#L505-L506

[JAndrassy]

DHCP sets the DNS server IP for specific interface_name. so they must be retrieved with the right interface_name.

for curios this is the implementation of get_dns_server in NetworkInterface.cpp in Mbed OS

nsapi_error_t NetworkInterface::get_dns_server(int index, SocketAddress *address, const char *interface_name)
{
    return get_stack()->get_dns_server(index, address, interface_name);
}