net: tcp: Fix user_data provided to accept callback

TCP implementation provided the parent net_context pointer to the
accept callback instead of the user_data pointer registered with
net_tcp_accept(). This worked fine with the socket integration, as
sockets explicitly registered parent context as user_data, however it
shouldn't be hardcoded like this at the TCP level.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>