net: sockets: tls: Allow handshake during poll()

When using DTLS socket, the application may choose to monitor socket
with poll() before handshake has been complete. This could lead to
potential crash (as the TLS context may have been left uninitialized)
and unexpected POLLIN reports (while handshake was still not complete).
This commit fixes the above - POLLIN will only be reported once
handshake is complete and data is available

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