iio: adc: ti-ads8344: properly byte swap value

The first received byte is the MSB, followed by the LSB so the value needs
to be byte swapped.

Also, the ADC actually has a delay of one clock on the SPI bus. Read three
bytes to get the last bit.

Fixes: 8dd2d7c0 ("iio: adc: Add driver for the TI ADS8344 A/DC chips")
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>