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

commit 279bf2e57c30c9a4482b2b6ede11b31c41e35e78 upstream.

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>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[herton: Backported to 3.4: info_mask was not used yet with another flag]
Signed-off-by: Herton R. Krzesinski <herton@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>