net/tcp: define a fallback value for rcv_wnd size

Some driver implements it's own network packet pool, so PKTBUFSRX is zero.
This results in zero-size TCP receive window, so data transfer doesn't
work. Avoid it by setting a reasonable fallback value.

Signed-off-by: Mikhail Kshevetskiy <mikhail.kshevetskiy@iopsys.eu>
Reviewed-by: Simon Glass <sjg@chromium.org>

net/tcp.c

@@ -36,7 +36,11 @@
 #define TCP_SEND_RETRY		3
 #define TCP_SEND_TIMEOUT	2000UL
 #define TCP_RX_INACTIVE_TIMEOUT	30000UL
-#define TCP_RCV_WND_SIZE	(PKTBUFSRX * TCP_MSS)
+#if PKTBUFSRX != 0
+  #define TCP_RCV_WND_SIZE	(PKTBUFSRX * TCP_MSS)
+#else
+  #define TCP_RCV_WND_SIZE	(4 * TCP_MSS)
+#endif
 
 #define TCP_PACKET_OK		0
 #define TCP_PACKET_DROP		1