staging: iio: ad5933: change attributes to match ABI

Change device attributes' names to match ABI documentation. Names were
chosen such that they tend to be similar to existing ABI so it should
be easier to standardize them when necessary.

Signed-off-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>