staging:iio:ade7758: Remove "raw" from channel name

commit b598aacc upstream.

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

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>