Bluetooth: hci_core: increase max adv inst

Now that all preconditions are present for actual multi-advertising, the
number of allowed advertising instances can be larger than one. This
patch increases the number of allowed advertising instances to 5.

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