staging:iio:ad7793: Fix scan index for the "shorted" channel

The code expects the scan index to match the offset of the channel into the
channel array. For the "shorted" the offset is 3, but the scan index is set to
2. Also the scan index 2 is already taken by the previous channel. As a result
the "shorted" channel will appear to be selected if the previous channel is
selected and vice versa and it is not possible to sample the "shorted" channel
in buffered mode.

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