Bluetooth: Host: add missing de-init of periodic sync pool

While execution of bt_disable there is missing de-initialization
of per_adv_sync_pool. If there was a sync in progress, (flags set
to BT_PER_ADV_SYNC_SYNCING) before bt_disable, then when the stack
is re-enabled by bt_enable new sync create fails.

The commit fixes the issue by new function that clears flags for
all per_adv_sync_pool entries. The function is executed by bt_disable.

Signed-off-by: Piotr Pryga <piotr.pryga@nordicsemi.no>