iio:accel:mma7455_core: Move exports into IIO_MMA7455 namespace

In order to avoid unnecessary pollution of the global symbol namespace
move the core mma7455 functions into an mma7455 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>
Cc: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20220116180535.2367780-3-jic23@kernel.org