iio: accel: kxsd9: Fix raw read return

Any readings from the raw interface of the KXSD9 driver will
return an empty string, because it does not return
IIO_VAL_INT but rather some random value from the accelerometer
to the caller.

Cc: stable@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>