Bluetooth: Host: conn_change signal object not correclty initialied

The problem is visible after call to bt_disable when there was
a connection. On a next bt_enable call when hc_tx_thread starts
it is possible that the conn_change object internals are not
re-initialized entirely and the function fails during k_poll call.

The commit changes explicit assignment conn_change object to
a k_poll_signal_init that does proper initialization.

Signed-off-by: Piotr Pryga <piotr.pryga@nordicsemi.no>