net: tcp: move accept_cb from net_context to net_tcp

net_context is used for more than just TCP contexts.  However,
the accept_cb field is only used for TCP.  Let's move it from
the generic net_context structure to the TCP specific net_tcp
structure.

Change-Id: If923c7aba1355cf5f91c07a7e7e469d385c7c365
Signed-off-by: Michael Scott <michael.scott@linaro.org>
4 files changed