staging:iio:ad7606: Re-add missing scale attribute

Commit 50ac23be ("staging:iio:adc:ad7606 add local define for chan_spec
structures.") accidentally removed the scale info_mask flag. This patch
adds it back again.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: stable <stable@vger.kernel.org>  [3.2+]
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>