iio: move 3-wire spi initialization to st_sensors_spi

Some devices need to be configured with special bit in order to
use spi 3-wire. This was done during device identification phase.
Instead, let's move this part as spi specific.
Doing this the check_device_support function becomes a simple
device id check, so let's rename it.

Signed-off-by: Denis Ciocca <denis.ciocca@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>