net: sockets: tls: Fix poll update event check

In case POLLIN is set, and no new application data has been detected,
the ztls_poll_update_ctx() should only return -EAGAIN if no other events
are available for the socket. Otherwise, the function may end up
busy-looping, in case for example POLLOUT is also monitored for the
socket.

Current check verifying that was wrong, as it caused to function to
return -EAGAIN even if some other events could've been reported for the
socket.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>
(cherry picked from commit bfe958a7)