staging:iio:adis16080: Cleanup SPI transfer

During sampling the driver currently does a spi_read followed by a spi_write.
This is not a problem per se, since CS needs to be deasserted between the two
transfers. So even if another device claims the bus between the two transfers we
should still get a result. But the code is actually spread out over multiple
functions. E.g. the spi_read happens in one function the spi_write in another,
this makes the code harder to follow. This patch re-factors the code to just use
a single spi transaction to do both the read and the write transfer.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>