staging:iio:ade7854: Rework I2C read function

staging:iio:ade7854: Rework I2C read function

The read operation for the I2C function has many duplications that can
be generalized into a single function. This patch reworks the read
operation for I2C to centralizes all similar code in a single function.

It is possible to remove all the old interface to use the new one,
however, for keeping the things simple and working this patch maintain
legacy interface.

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