Bluetooth: hci_uart: Use h4_recv_buf helper for Atheros AR300x

Instead of using hci_recv_stream_fragment, use the local available
h4_recv_buf helper function.

To ensure that the function is available select BT_HCIUART_H4.

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