rpmsg_sockif: recv/recvmsg return 0 when peer close

Follow the posix standard:
If no messages are available to be received and the peer has
performed an orderly shutdown, recv() shall return 0.

Signed-off-by: Bowen Wang <wangbowen6@xiaomi.com>

net/rpmsg/rpmsg_sockif.c

@@ -1301,7 +1301,7 @@ static ssize_t rpmsg_socket_recvmsg(FAR struct socket *psock,
                       _SO_TIMEOUT(conn->sconn.s_rcvtimeo));
   if (!conn->ept.rdev || conn->unbind)
     {
-      ret = -ECONNRESET;
+      ret = 0;
     }
 
   nxmutex_lock(&conn->recvlock);