Bluetooth: hci_raw: Fix buffer init after allocation

The code was not properly taking into account CONFIG_BT_HCI_RESERVE,
which would cause buffer underruns for any HCI driver where this value
defaults to non-zero. Also, all the allocation functions use the same
pool, so we can map them simply to bt_buf_get_rx() instead of
repeating the same code.

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