Commit 3f442d3 ("[tcp] Record ts_recent on first received packet")
failed to achieve its stated intention.

Fix this (and reduce the code size) by moving the ts_recent update to
tcp_rx_seq().  This is the code responsible for advancing the window,
called by both tcp_rx_syn() and tcp_rx_data(), and so the window check
is now redundant.

Reported-by: Frank Weed <zorbustheknight@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>