staging:iio:dac:ad5791: Move private structs and defines from header to C file

The header for this driver contains some private structs and defines, which do
not have to be shared between multiple source files, as well as the platform
data struct for this driver, which has to be shared with other source files.
Since there is no need to expose those private structs and defines move them to
the source file.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>