iio: adc: ti-ads1015: Switch to static const writeable ranges table

Switch the driver from code implementing test whether a regmap register
is writeable to static const tables describing the test. No functional
change.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Cc: Daniel Baluta <daniel.baluta@nxp.com>
Link: https://lore.kernel.org/r/20220328194725.149150-3-marex@denx.de
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>