net/tcp/tcp_send_buffered.c: Fix non-blocking I/O

My recent changes to buffered tcp send broke this. [1]

One of my local apps using non-blocking tcp is working
again with this fix.

[1]
```
commit 837e1a72a47b4e5a874124e316192b2172301d30
Author: YAMAMOTO Takashi <yamamoto@midokura.com>
Date:   Mon Mar 15 16:19:42 2021 +0900

    tcp_send_buffered.c: improve tcp write buffering
```

net/tcp/tcp_send_buffered.c

@@ -1259,6 +1259,11 @@ ssize_t psock_tcp_send(FAR struct socket *psock, FAR const void *buf,
       if (chunk_result == 0)
         {
           DEBUGASSERT(nonblock);
+          if (result == 0)
+            {
+              result = -EAGAIN;
+            }
+
           break;
         }