SFTP Fix

1. Remove the continue from the SFTP worker loop when there's a timeout
   on the select and when there's a want_read from the wolfSSH_worker.
2. When the select has receive data ready, check it. Don't make it
   conditional on want read as well.

The goal is that when there isn't data waiting on the socket, check and
process the data in the SFTP channel's buffer, always.

examples/echoserver/echoserver.c

@@ -1196,11 +1196,8 @@ static int sftp_worker(thread_ctx_t* threadCtx)
         }
         else if (selected == WS_SELECT_TIMEOUT) {
             timeout = TEST_SFTP_TIMEOUT_LONG;
-            continue;
         }
-
-        if (ret == WS_WANT_READ || ret == WS_WANT_WRITE ||
-                selected == WS_SELECT_RECV_READY) {
+        else if (selected == WS_SELECT_RECV_READY) {
             ret = wolfSSH_worker(ssh, NULL);
             error = wolfSSH_get_error(ssh);
             if (ret == WS_REKEYING) {
@@ -1213,7 +1210,6 @@ static int sftp_worker(thread_ctx_t* threadCtx)
                 error == WS_WINDOW_FULL) {
                 timeout = TEST_SFTP_TIMEOUT;
                 ret = error;
-                continue;
             }
 
             if (error == WS_EOF) {