drivers: bluetooth: hci: Remove header acquisition from bt_spi_send

Replace the header acquisition scope in bt_spi_send with bt_spi_get_header.

Add WRITE_DATA_CONDITION in order not to retransmit data without reading
the header as it is meaningless for ST BlueNRG devices.

Signed-off-by: Ali Hozhabri <ali.hozhabri@st.com>