Bluetooth: Move pending management command list into struct hci_dev

This patch moves the pending management command list (previously global
to mgmt.c) into struct hci_dev. This makes it possible to do proper
locking when accessing it (through the existing hci_dev locks) and
thereby avoid race conditions.

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>