iio:adc:ad7091r: Move exports into IIO_AD7091R namespace.

In order to avoid unnecessary pollution of the global symbol namespace
move the core/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/



An alternative here would be to conclude that we are unlikely to see
support for the other ad7091r parts in the near future and just merge
the two modules into one supporting just the i2c -5 variant.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220130205701.334592-3-jic23@kernel.org