iio:magnetometer:bmc150: Move exports to IIO_BMC150_MAGN namespace

iio:magnetometer:bmc150: Move exports to IIO_BMC150_MAGN namespace

In order to avoid unnecessary pollution of the global symbol namespace
move the common/library functions into a specific namespace and import
that into the bus specific device drivers that use them.

Note the MAGN postfix here is reflecting that this driver is only
responsible for part of the BMC150 device.

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

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220130205701.334592-14-jic23@kernel.org