staging: iio: ad9834: Moved contents of the header to the source file

Moved the contents of the header(ad9834.h) into the corresponding source
file with the exception of the platform data struct which is supposed to be
used from somewhere else other than the driver.

Signed-off-by: Narcisa Ana Maria Vasile <narcisaanamaria12@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>