Bluetooth: Refactor hci_dev_open to a separate hci_dev_do_open function

The requirements of an external call to hci_dev_open from hci_sock.c are
different to that from within hci_core.c. In the former case we want to
flush any pending work in hdev->req_workqueue whereas in the latter we
don't (since there we are already calling from within the workqueue
itself). This patch does the necessary refactoring to a separate
hci_dev_do_open function (analogous to hci_dev_do_close) but does not
yet introduce the synchronizations relating to the workqueue usage.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>