drivers: bluetooth: hci: spi: Handle when the buffer is empty

Check if the buffer size is not empty before spi transceive and buffer's
data processing. This fixes the need to slow down MCUs to 16MHz when
using BlueNRG-MS chips.
Tested samples : Beacon, Central, Peripheral

Signed-off-by: Yaël Boutreux <yael.boutreux@st.com>
Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>