iio:accel:adxl313: Move exports into IIO_ADXL313 namespace

In order 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>
Cc: Lucas Stankus <lucas.p.stankus@gmail.com>
Link: https://lore.kernel.org/r/20220116180535.2367780-6-jic23@kernel.org