iio: make function argument and some structures const

Make the argument of the functions iio_sw{d/t}_group_init_type_name const
as they are only passed to the function config_group_init_type_name having
the argument as const.

Make the config_item_type structures const as they are either passed to
the functions having the argument as const or they are
stored in the const "ci_type" field of a config_item structure.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@Huawei.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>