iio:accel:mma9551_core: Move exports into IIO_MMA9551 namespace

In order to avoid unnecessary pollution of the global symbol namespace
move the core mma9551 functions into an mma9551 specific namespace.

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



Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20220116180535.2367780-2-jic23@kernel.org