Bluetooth: mgmt: Fix pairable setting upon initialization

When mgmt is not in use the HCI_PAIRABLE flag will get implicitly set so
that pairing still works with old user space versions. However, as soon
as mgmt comes into play we should clear this flag so that it can be
properly set through the set_pairable command by user space.

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