staging:iio:ad7887: 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 and we also always build buffer support,
so there really is no need to spread the driver out over multiple files. Putting
everything into one file also allows to reduce the code size a bit more by
removing a few lines of boilerplate code.

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