Bluetooth: Add missing hci_dev locking when calling mgmt functions

Now that the pending commands are within struct hci_dev we can properly
control access to them throught the hci_dev locking mechanism.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>