staging:iio:adis16400: Code style cleanup

Do a set of minor miscellaneous code style cleanups for the adis16400 before
moving it out of staging. Delete outdated comments, removed excess whitespace,
add missing whitespace, replace u{8,16} with uint{8,16}_t.

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