Bluetooth: SMP: Fix crash when receiving new connection when debug is enabled

When receiving a new connection pchan->conn won't be initialized so the
code cannot use bt_dev_dbg as the pointer to hci_dev won't be
accessible.

Fixes: 2e1614f7

 ("Bluetooth: SMP: Convert BT_ERR/BT_DBG to bt_dev_err/bt_dev_dbg")
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>