staging:iio:ad7298: Squash everything into one file

The recent cleanups have decimated the drivers code size by quite a bit. It is
only a few hundred lines in total now. Putting everything into one file also
allows to reduce the code size a bit more by removing a few lines of boilerplate
code. The only functional change made by this patch is that we now always
include buffer support, instead of making it optional. This is more consistent
with what we do for other drivers.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>