Bluetooth: Enable passive scanning whenever we're connectable

Enabling passive scanning always when we're connectable aligns us with
the BR/EDR page scanning. This is also consistent with the fact that the
code dealing with passive scanning results will actively try to connect
any direct advertising event when we're connectable.

This patch implements the feature by adding the connectable condition to
hci_update_background_scan() checks for starting scanning and by calling
hci_update_background_scan() whenever the connectable state changes.

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