staging:iio:ad799x: Set IIO_CHAN_INFO_SCALE mask

The driver has support for reporting a channels scale, but none of the channels
set the IIO_CHAN_INFO_SCALE mask. Add the IIO_CHAN_INFO_SCALE to all channels to
indicate that the drivers supports reporting the scale.

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