staging:iio:ade7854: Remove read_reg_* duplications

staging:iio:ade7854: Remove read_reg_* duplications

The original code had a read function per data size; after updates, all
read functions tasks were centralized in a single function, but the old
signature was kept to maintain the module working without problems. This
patch removes a set of duplications associated with read_reg_*, and
update the areas that calling the old interface by the new one.

Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>