iio:common:ssp_sensors: Move exports into IIO_SSP_SENSORS 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 various specific device drivers that use them.

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



Both the exports used between the two common modules and the individual
drivers are moved to a single namespace as greater granularity does
not feel useful.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220130205701.334592-6-jic23@kernel.org