net: tcp: Report TCP errors through recv_cb()

Make use of the status argument in the recv_cb() callback function -
instead of blindly reporting ECONNRESET whenever TCP context is
dereferenced, indicate whether an actual error condition happened (by
setting respective errno value) or a graceful shutdown took place (by
setting status to 0).

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