[tftp] Abort requests with error code 0

There is no defined error code for aborting a request but 0 is commonly
used.  This patch switches the abort request error code from
TFTP_ERR_UNKNOWN_TID (5) to 0.

Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Marty Connor <mdc@etherboot.org>

src/net/udp/tftp.c

 	/* Abort request if only trying to determine file size */
 	if ( tftp->flags & TFTP_FL_SIZEONLY ) {
 		rc = 0;
-		tftp_send_error ( tftp, TFTP_ERR_UNKNOWN_TID, "TFTP Aborted" );
+		tftp_send_error ( tftp, 0, "TFTP Aborted" );
 		tftp_done ( tftp, rc );
 		return rc;
 	}
 	if ( tftp->flags & TFTP_FL_SIZEONLY ) {
 		/* If we get here then server doesn't support SIZE option */
 		rc = -ENOTSUP;
-		tftp_send_error ( tftp, TFTP_ERR_UNKNOWN_TID, "TFTP Aborted" );
+		tftp_send_error ( tftp, 0, "TFTP Aborted" );
 		goto done;
 	}