iio:accel:adxl345: Move exports into IIO_ADXL345 namespace

To avoid unnecessary pollution of the global symbol namespace move the
driver core exports into their own namespace and import that into the two
bus modules.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20220116180535.2367780-7-jic23@kernel.org