AD7887_CH0_CH1
spi_message_init(&st->msg[AD7887_CH0_CH1]);
spi_message_add_tail(&st->xfer[1], &st->msg[AD7887_CH0_CH1]);
spi_message_add_tail(&st->xfer[2], &st->msg[AD7887_CH0_CH1]);
st->ring_msg = &st->msg[AD7887_CH0_CH1];