Bluetooth: HCI: Use H:4 encoding for buffers

Encode the packet type as a H:4 payload prefix for buffers passing to &
from HCI drivers. The existing bt_buf_set/get_type functions are
deprecated, but kept compatible with the change, except that they can only
be called once, since they modify the buffer payload.

Signed-off-by: Johan Hedberg <johan.hedberg@silabs.com>