iio:accel:fxl8962af: Move exports into IIO_FXL8962AF 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.

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



Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Sean Nyekjaer <sean@geanix.com>
Reviewed-by: Sean Nyekjaer <sean@geanix.com>
Link: https://lore.kernel.org/r/20220116180535.2367780-12-jic23@kernel.org