staging: iio: ad2s1210: Fix SPI reading

If the serial interface is used, the 8-bit address should be latched using
the rising edge of the WR/FSYNC signal.

This basically means that a CS change is required between the first byte
sent, and the second one.
This change splits the single-transfer transfer of 2 bytes into 2 transfers
with a single byte, and CS change in-between.

Note fixes tag is not accurate, but reflects a point beyond which there
are too many refactors to make backporting straight forward.

Fixes: b19e9ad5 ("staging:iio:resolver:ad2s1210 general driver cleanup.")
Signed-off-by: Dragos Bogdan <dragos.bogdan@analog.com>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>