net: sockets: tls: Timeout DTLS with poll

Changed poll to perform incoming data check with connected
DTLS connections. This allows the CONFIG_NET_SOCKETS_DTLS_TIMEOUT
to timeout the connections to server socket if there is no
incoming data.

Previously, if the remote client closed the DTLS connection without
close notify, the timeout of the ongoing connection only happened
when next connection was taken to the server socket. Depending on
the timeouts, this could prevent the next connection from succeeding.

Signed-off-by: Markus Lassila <markus.lassila@nordicsemi.no>