net: conn: Move net_conn_change_callback() to static

The net_conn_change_callback() is not currently being called by
anyone, so this commit moves to static function, and replaces
the change callback parameter process in net_conn_register().

Signed-off-by: Takuya Sasaki <takuya.sasaki@spacecubics.com>
(cherry picked from commit 49c6da51)