staging:iio:ad5933: Drop "raw" from channel names

commit 6822ee34 upstream.

"raw" is the name of a channel property, but should not be part of the
channel name itself.

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