staging:iio:dummy: Register same channels for device and buffer

In preparation for moving the buffer registration to the core make sure to
register the same channel array for the device and the buffer. Currently the
output voltage and the activity channels are not registered for the buffer,
setting its scan index to -1 will make sure that it is skipped for the
buffer.

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