iio: adc: ti-ads1015: Switch to read_avail

Replace sysfs attributes with read_avail() callback. This also permits
removal of ads1115_info, since the scale attribute tables are now part
of chip data.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Daniel Baluta <daniel.baluta@nxp.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Link: https://lore.kernel.org/r/20220328194725.149150-10-marex@denx.de
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>