staging:iio:accel:sca3000: Fix off by one error in axis due to IIO_NO_MOD

Given the introduction of IIO_NO_MOD was prior to the first submission
prior to IIO entering staging this has been broken for a while.

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