drivers: bluetooth: hci: Restructure spi.c to handle different protocols

Update structure of spi.c to have a better and cleaner separation
between STMicroelectronics and Zephyr SPI protocol.

Introduce bt_spi_get_header to separate algorithms well for
header acquisition in different protocols.

Remove header acquisition from bt_spi_rx_thread to make it simpler.

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