Bluetooth: Use HCI_EV_HARDWARE_ERROR define for event payload

When constructing the event payload for the HCI_Hardware_Error event
message, use the HCI_EV_HARDWARE_ERROR define.

In addition rename the variables from hard_err to hw_err to clearly
indicate that this is about the hardware error and not a hard error.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>