tcpip_poll_tcp() was changed with commit 61edc68c to take on a
data_buf parameter, which was then processed as the primary
buffer. That change led to incorrect behaviour where the handling
of the first data buffer on a connection got mixed with the SYN buf.
It is no longer clear why tcpip_poll_tcp() was modified with the
change 61edc68c originally. Reverting the modification to
tcpip_poll_tcp() leads to much better handling of TCP data; and
also obviates another pull-request submitted recently:
https://gerrit.zephyrproject.org/r/#/c/4226.
Change-Id: I947c0991495c538c41e6581c8d360526b1bb89ad
Signed-off-by: Rohit Grover <rohit.grover@arm.com>
769e2541d1