samples: hci_spi: Fix cmd_hdr and acl_hdr usage

Fixes #27266.
cmd_hdr and acl_hdr local structs were read but never written.
Causing unpredictable errors in bt_tx_thread such as :
"Invalid HCI CMD packet length" in hci.c:2280
or Imprecise data bus errors on nrf52810.

cmd_hdr and acl_hdr are now filled with the received data
from SPI Master.

Signed-off-by: Corentin Dorothée <corentin.dorothee@gmail.com>