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

commit 6822ee34ad57b29a3b44df2c2829910f03c34fa4 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>
[bwh: Backported to 3.2: using IIO_CHAN() macro to initialise the structures]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>