staging:iio:ad799x: Move ring functions to the main file

The ad799x_ring.c file is pretty much only one function these days. No need to
keep it in a separate file. Since there is then only one user of the header left
also move everything from the header to the main file.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>