iio:accel:kxsd9: Move exports into IIO_KDSD9 namespace

In order to avoid unnecessary pollution of the global symbol namespace
move the core kxsd9 functions into a kxsd9 specific namespace and import
that into the two bus modules.

For more information see https://lwn.net/Articles/760045/



Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20220116180535.2367780-4-jic23@kernel.org