[tcp] Fix comment to match code behaviour

Reported-by: Thomas Miletich <thomas.miletich@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/tcp.c

 	tcp_dump_flags ( tcp, tcphdr->flags );
 	DBGC2 ( tcp, "\n" );
 
-	/* If no connection was found, send RST */
+	/* If no connection was found, silently drop packet */
 	if ( ! tcp ) {
 		rc = -ENOTCONN;
 		goto discard;