samples: net: echo_client/server: Fix mbed TLS dependency

All application level TLS operations should be guarded with
"#if defined(CONFIG_NET_SOCKETS_SOCKOPT_TLS)" and not mbed TLS configs
only. Otherwise, in case mbed TLS is enabled, but the sample is not
configured to use TLS (hence TLS credential library is not available
for example), the build may fail or produce spurious warnings.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>