examples/usrsocktest: read from the closed remote should return EOF

fix usrsock remote_disconnect fail:

nsh> usrsocktest
...
Testing group "remote_disconnect" =>
        [TEST ASSERT FAILED!]
                In function "receive":
                line 497: Assertion `(ssize_t)((ret)) == (ssize_t)((0))' failed.
                        got value: -1
                        should be: 0
        Group "remote_disconnect": [FAILED]
...

Reference:

RECV(2)

NAME
       recv, recvfrom, recvmsg - receive a message from a socket
...
RETURN VALUE
...
       When a stream socket peer has performed an orderly shutdown,
       the return value will be 0 (the traditional "end-of-file" return).

       Datagram sockets in various domains (e.g., the UNIX and Internet domains)
       permit zero-length datagrams.  When such a datagram is received, the return value is 0.

Signed-off-by: chao an <anchao@xiaomi.com>

examples/usrsocktest/usrsocktest_daemon.c

@@ -856,7 +856,7 @@ static int recvfrom_request(int fd, FAR struct daemon_priv_s *priv,
 
   if (!tsock->connected)
     {
-      ret = -ENOTCONN;
+      ret = (tsock->endp) ? 0 : -ENOTCONN;
       goto prepare;
     }