TCP window scaling: change tcp_pcb::acked to u32_t when window scaling is enabled to prevent an overflow when more than 64 Kbytes are ACKed

src/include/lwip/tcp.h

@@ -244,7 +244,7 @@ struct tcp_pcb {
   tcpwnd_size_t snd_wnd;   /* sender window */
   tcpwnd_size_t snd_wnd_max; /* the maximum sender window announced by the remote host */
 
-  u16_t acked;
+  tcpwnd_size_t acked;
 
   tcpwnd_size_t snd_buf;   /* Available buffer space for sending (in bytes). */
 #define TCP_SNDQUEUELEN_OVERFLOW (0xffffU-3)