Bluetooth: Add flag to track the real advertising state

Having a single HCI_ADVERTISING flag is problematic since it tries to
track both the real advertising state and the corresponding mgmt
setting. To make the logic simpler and more reliable add a new flag that
only tracks the actual advertising state that has been written to the
controller.

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