iio: accel: kxsd9: fix the usage of spi_w8r8()

These two spi_w8r8() calls return a value with is used by the code
following the error check. The dubious use was caused by a cleanup
patch.

Fixes: d34dbee8 ("staging:iio:accel:kxsd9 cleanup and conversion to iio_chan_spec.")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>