iio: accel: kxsd9: Fix raw read return

commit 7ac61a062f3147dc23e3f12b9dfe7c4dd35f9cb8 upstream.

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.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
[bwh: Backported to 3.2: adjust filename]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>