staging:iio:ad7476: Squash driver into a single file.

After the recent cleanups the buffer support is just a single 23 line function.
This does not really justify a file on its own, so move it to the main driver
file. And with only one source file left the header file containing the device
state struct becomes superflousious so move the content of the header
file to the main driver source file as well.

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