staging:iio:ade7854: Remove write_reg_* duplications

This patch removes code duplications related to the write_reg_*
functions and centralizes them in a single function. Also, it eliminates
the legacy functions and replaces them by a unique signature that is
used by SPI and I2C.

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