net/tcp: define a fallback value for rcv_wnd size

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>