staging:iio:adc:ad7152: update scale handling

Add scale_available attribute.
fix ad7152_scale_table, values are not sorted descending.
Use IIO_VAL_INT_PLUS_NANO, to increase granularity.
Update scale handling accordingly.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>