staging:iio:ade7854: Rework I2C write function

The write operation using I2C has many code duplications and four
different interfaces per data size. This patch introduces a single
function that centralizes the main tasks.

The central function inserted by this patch can easily replace all the
four functions related to the data size. However, this patch does not
remove any code signature for keeping the meter module work and make
easier to review this patch.

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