Bluetooth: hci_core/mgmt: move adv timeout to hdev

Currently the delayed work managing advertising duration and timeout is
part of the advertising instance structure. This is not correct as only
a single instance can be advertised at any given time. To implement
round robin advertising a single delayed work structure is needed.

To fix this the delayed work structure is being moved to the hci_dev
structure. The instance specific variable is renamed to "remaining_time"
to make it clear that this is the remaining lifetime of the instance and
not the current advertising timeout.

Signed-off-by: Florian Grandel <fgrandel@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>