staging:iio:ade7854: Rework SPI write function

The write operation using SPI has a many code duplications (similar to
I2C) and four different interfaces per data size. This patch introduces
a single function that centralizes the main task related to SPI.

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