staging: iio: ad5933: move contents of header file to source file

The contents of the header file are used only by this single
source file.  Move content into .c and remove .h.

Signed-off-by: Alison Schofield <amsfield22@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>