staging:iio:ad7291: Move out of staging

The ad7291 driver is in a reasonable shape. It does not use non-standard API/ABI
and there are no major style issues with the driver. So this patch moves it out
of staging.

There is one small warning from checkpatch which is also fixed in this patch.
The patch also sorts the #include directives in alphabetical order.

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