Bluetooth: Refactor HCI command skb creation

This patch moves out the skb creation from hci_send_cmd() into its own
prepare_cmd() function. This is essential so the same prepare_cmd()
function can be easily reused for skb creation for asynchronous HCI
requests.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>