net: sockets: tls: Fix ZSOCK_POLLHUP detection

The previous approach to detect if the underlying transport was closed
(by checking the return value of `mbedtls_ssl_read()` was not right,
since the function call does not request any data - therefore 0 as a
return value is perfectly fine.

Instead, rely on the underlying transport ZSOCK_POLLHUP event - if it
reports that the connection ended, forward the event to the application.

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