iio: Simplify iio_map_array_unregister API

Instead of requiring the map to unregister, simply unregister all map entries
associated with the given iio device. This simplifies map removal and also works
for maps generated through devicetree.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>