Bluetooth: Fix re-enabling advertising after a connection

LE controllers will automatically disable advertising whenever they
accept a new connection. In order not to fall out of sync with the
advertising setting we need to re-enable advertising whenever the last
LE connection drops. A failure to re-enable advertising should cause the
setting to be disabled, so this patch also calls mgmt_new_settings()
when this happens.

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