Bluetooth: Use zalloc when possible

Use zallog for adv_instance allocation instead of kmalloc + memset.
This also fixes the following coccinelle warning:

>> net/bluetooth/hci_core.c:2693:17-24: WARNING: kzalloc should be
used for adv_instance, instead of kmalloc/memset

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