Bluetooth: Rename pending_cmd to mgmt_pending_cmd

This patch renames the pending_cmd struct (used for tracking pending mgmt
commands) to mgmt_pending_cmd, so that it can be moved to a more generic
place and be used also by other modules using other HCI channels.
Signed-off-by: NJohan Hedberg <johan.hedberg@intel.com>
Signed-off-by: NMarcel Holtmann <marcel@holtmann.org>